The Pamphlet Collection of Sir Robert Stout: Volume 67
Description of the Great Fissure
Description of the Great Fissure.
Map No. 2, attached to this report, is based on the recent topographical survey, made specially to illustrate the changes in the country which have taken place since the eruption. It will be seen that the result of such eruption is the formation of a fissure in the earth's crust running nearly straight on a true bearing of 58° for a distance of eight miles and three-quarters, extending from the north end of Wahanga to near Okaro Lake. South-west of this it is continued by earthquake-cracks for some miles further. The fissure varies considerably in width and depth, being widest at Rotomahana, where it has a maximum breadth of a mile and a half, whilst its general or mean width where it crosses the mountain may be stated as an eighth of a mile. The depth is greatest near the northern end, just between Wahanga and Ruawahia, where it is about 900ft. below "the gap," or 1,400ft. below the top of Ruawahia; and the least is at the southern end, where it may be said approximately to have a depth of 300ft. It must not, however, be supposed that the fissure has the appearance of a continuous rent or split in the earth's surface for its whole length; on the contrary, it is bridged in several places by parts of the original surface remaining in position. It presents, indeed, more the character of a series of craters (not necessarily circular), arranged in a nearly straight line and at various elevations above sea-level.
As it will be necessary to use the word "crater" very frequently, it is as well to explain that it is intended to convey the idea of orifices of very different characters, but all of which have been actively engaged in the ejection of solid matter, either in the form of molten rock and scoria; or sand, earth, mud, and unfused rock, accompanied in the latter case usually by water. The first description of crater is confined to that part of the fissure extending from the north end of Wahanga to the base of Tarawera; the second description extends from the north end of Rotomakariri to the Southern Crater; whilst the Black Crater combines the characteristics of both.page 46
Standing on the highest point of Ruawahia, 3,770ft. above sea-level, and looking north-east, the fissure is seen to have passed along the steep wall-like face of Wahanga, cutting part of it off, and terminating near the northern end of that hill in a deep oblong crater, the eastern side of which is very much lower than the western. Immediately to the north of Ruawahia is a long deep crater, with nearly perpendicular sides, which cuts off "the gap" completely, and the western side of which is also much higher than the eastern, reaching tip, in fact, to the level of the Wahanga Plateau, whilst the opposite side is but little elevated above the lower plateau extending to Kanakana. The bottom of this fearful-looking chasm is about 1,100ft. below the top of Ruawahia. Its sides are so steep that a descent into it would be impossible. It is scorched and blackened by the heated matter from below, though here and there the rugged faces of the original lighter-coloured rock can be seen. The solid contents of it appear to have been blown clean out, and are scattered all over the country in fragments of various sizes, the larger and first-ejected portions being doubtless hidden by the black and reddish scoria which is now piled up on its edges. Some of this has fallen back again into the vent, and thus prevents a clear view of the more solid lava which is probably present at its bottom. It is separated from the most northern crater by a sharp ridge composed of loose scoria, which has fallen back from the air after being ejected, and which, no doubt, marks a point in the fissure where the molten matter met with some obstruction, or where activity first ceased. The lower walls of this crater are quite perpendicular, whilst the upper parts slope at a high angle to the surface. When the Survey party was there on the 28th July it was extinct, excepting that on the western side, near the Wahanga Plateau, steam escaped from a number of cracks, as it also did just outside the crater-edge on the eastern side. It is difficult to estimate the depth of newly-deposited scoria along the edges of these two craters; but it appears to be about 40ft. or 50ft., decreasing in depth as the crater is left.
The margins of the fissure have several little peaks of scoria, rising as much as from 50ft. to 100ft. above the general level, and the outer slopes of these descend at a considerable inclination to the lower parts of the plateau. (See Plate No. 4A.) Their structure is very evident to the eye, and illustrates exactly the ordinary formation of volcanic cones by the building-up of outwardly-inclined strata of loose scoria mixed with larger fragments of the unfused native rock. Inside the fissure the ejected scoria has fallen back so as to leave a steep talus sloping down to the bottom of the craters, though in many places the original rock can be seen protruding through it, sometimes partly fused and become scoriaceous, at others apparently unaffected by the passage of the heated matter.
Plate No. 5, looking along the fissure southerly, and across the deepest crater, shows very distinctly the surface of the older rocks, distinguished by their lighter colour; and here the deposit of new matter varies from 70ft. to 120ft. in depth. It will be seen also that this, the deepest crater, has much more perpendicular walls than the others. The margins of the fissures are cracked everywhere by minor fissures running generally parallel to the main one, though some are at right angles to it. In a few of the little peaks these cracks follow nearly horizontal lines round them, one above another, indicating the probability of a subsidence of the whole mass into the crater at no distant date. Nearly everywhere the regular stratification of the scoria along the margins can be traced, but it is particularly marked in the rounded southern end of this part of the fissure, as illustrated in Plate No. 6.page 48
The surface of the slopes is everywhere composed of fine scoria of a black or reddish-brown colour, intimately mixed with fine pumiceous sand; and scattered over it in great profusion are blocks of trachytic and rhyolitic rock, of sizes varying from a few inches in cubic contents to blocks measuring as much as 250 to 300 cubic feet. Most of these show little or no sign of fusion; but there is one place on the south-east side of the fissure where enormous masses of fused or partly-fused rock (lava, in fact) are to be found. Two pieces of rounded form, containing about 250 cubic feet each, were found partly imbedded in the fine scoria, and these had evidently been ejected after the finer material on which they rested, for in their fall they had excavated dish-like hollows, and broken up other rocks on which they fell. Both were completely molten on the surface. Nor were these the only rocks which showed by their position the date of their ejection. Numbers of others, and many of large size, have certainly been thrown up since the outburst ceased in any force. Several minor outbursts of short duration were witnessed from Rotorua and other places during the first and second weeks after the eruption, and these rocks were doubtless cast out at that time. The Survey party witnessed from below one of these paroxysmal outbursts on the 6th August. Without any warning or noise a vast mass of steam and smoke shot suddenly into the air with great velocity to a height of a thousand feet or more, and gradually rolled away before the wind. It was like the smoke from a gun, and was over directly.*
* As recently as the 30th October a considerable body of smoke was observed to rise from Wahanga.
Drawn by G. N. Sturtevant from a photo: by C. Spenser.
Shewing old surface of Mountain.
One of the craters in the Great Fissure on top of Ruawahia and Tarawera,
shewing the stratified deposits of scoria and ejected stone; these latter are colored a brilliant yellow by ferric chloride. (Looking South West).
When the first clear view of the mountain was obtained a few days after the eruption, the top and sides were seen to be coloured green and yellow, which gave rise to the report that sulphur in great quantities had been deposited there. This appearance was caused by the deposition of ferric chloride in large amount. The colour has since changed to reddish brown. A few minute crystals of sulphur were found in one locality, but only in one.
No sign of any lava-stream having issued from the fissure is to be seen anywhere. Solid lava may be, and probably is to be, found at the bottom of the craters, but at a considerable depth; but it certainly has not flowed out: and therefore those who saw the outburst on the 10th June are mistaken in supposing they saw lava pouring slowly down the mountain-side. The steam or vapour which rises from the craters and the cracks along their margins is that of muriatic acid—its pungent odour is unmistakable; but in some parts a strong smell of iodine was also noticed, and in others an offensive scent like that of burnt leather. As to what this may be, the writer hesitates to offer an opinion.
The escape of vapour, however, is not confined to the fissure or its immediate vicinity: places as far off as half a mile—some near the edge of the plateau—were also emitting steam in small quantities, which seems to indicate that the mountain has been shaken and fractured to its very base, thus allowing the heated vapour to escape by the crevasses formed. This opinion is also supported by the fact of finding the sands at the base of the mountain everywhere warm at 6in. under the surface, even after heavy rains. On the fifth day after the eruption the writer observed with the telescope that after each shock of earthquake—which then occurred hourly—cracks opened on the side and base of the mountain, and at considerable distances from the fissure, from which little jets of steam arose—to disappear, however, after the lapse of a few minutes. Generally the vapour is quite white in colour; but every now and then a jet of reddish-brown smokelike steam is sent up from the craters, and in one or two places steam of a very light-bluish colour was noticed.
At the southern end of the fissure on Mount Tarawera one of the breaks or bridges occurs, where the escaping steam does not appear to have had sufficient power to force an outlet through the rocks of the mountain. The steep slope which this bridge occupies is covered page 50 thickly with rocks of all sizes ejected from the fissure above, the space between being filled up with fine scoria and sand. The latter is, doubtless, derived from Rotomahana and that part of the fissure which lies on the north-east of it. This slope terminates on the perpendicular south-west face of Tarawera. In several places on it steam was escaping on the 28th July, showing that, although unequal to the exertion of opening a great fissure continuously, the eruption has cracked and shaken the underlying rocks sufficiently to allow the steam to reach the surface. The bridge is just a quarter of a mile in width. Its southern end marks the higher edge of the great crater forming the northern termination of the chasm which extends from top to bottom of the south-west side of Tarawera, the depth of which is between 800ft. and 900ft. This depth decreases as the lower end of the chasm is reached, until it finally runs out into the sloping plain lying at the base of the mountain. The western edge is very sharply defined—the rocks falling almost perpendicularly to the bottom—and is, like the fissure on the top of the mountain, a good deal cracked and shaken: the cracks run, however, more at right angles to the chasm than they do above. On the eastern side, nearly half-May down, another crater has been formed, which widens the fissure out somewhat, but its average breadth is about 80yd. Here, again, muriatic-acid fumes are escaping from the cracks, with the yellow ferric-chloride deposits lining them just as above. At the end of July but little activity was shown along this part of the fissure, though every now and then a column of reddish smoke was sent up into the air, but in no great volume. Plates Nos. 7 and 8, taken from below, show the appearance of the chasm.
Plate No 27.
Drawn by G. N. Sturtevant from a photo: by C. Spenser.
in the south-west end of Tarawera Mountain, with Green Lake Crater.
The steep slopes of Tarawera and Ruawahia Mountains, which lie directly under the "mural crown," are covered thickly with fine scoria, which, in many instances, has already become consolidated into a breccia which is difficult and even dangerous to travel over where the inclination is great. The cementing-matter appears to be the very fine sand or mud (or ash) whose origin is probably Rotomahana. The gently-sloping plain south of the foot of Tarawera is thickly covered with this scoria, as are the hills to the east, and where the water has already cut through it and spread it along the channels, it forms a hard stony bed just like the breccias seen in older volcanic formations, which are noted for their hardness and power of resisting atmospheric denudation.
* The writer makes no pretence to a knowledge of mineralogy, any more than such a rough one as may be gained by the comparison of specimens with the type-rock usually found in every museum.
Mention has been made of the forests which once clothed the slopes of these mountains: they were of considerable extent, especially on the south-eastern flanks. The eruption has utterly destroyed them over an area of many square miles. They present the most melancholy and desolate aspect: not a green leaf is to be seen anywhere, but, instead, a mass of broken limbs and riven trunks, wrenched and torn to an extent that renders the wood utterly useless. Many of the trees have been burnt, and their charred remains alone are to be seen. The slope now occupied by the chasm on the south-west side of Tarawera was partly under forest formerly, and the trees which covered it suffered in the same manner as the rocks, and have been blown away far and wide over the country. On the southern side of Rotomahana large logs, some as much as 3ft. in diameter by 15ft. and 20ft. long, are found half-buried in the sands, battered and worn, and sodden with water; and these must have come from Tarawera, as there was no other forest in the line of eruption. They have been hurled through the air a distance of four miles. A very striking instance of the force of the wind which accompanied the eruption is exhibited by a totara tree, the stump of which is still standing near the chasm. It protrudes from the ground for about 15ft. or 18ft., and is evidently standing on the spot where it grew: at that height from the ground it is about 8ft. in diameter. The top has been twisted off, and has disappeared, leaving the jagged stump with the splinters all directed in the same way, showing it to have been subjected to a violent wrench. Many other trees (principally rimus) have suffered in a similar manner in the same locality.
* See Appendix, for note on recently-formed obsidian.
The craters and points of eruption which occupy the line of fissure from Wahanga to the base of Tarawera belong to that class which have been described above as characterized by the ejection of molten rock or scoria. But in continuing the description of the fissure towards the south-west we shall deal exclusively—the Black Crater excepted—with the other class, or that in which the ejection of unfused rock, mud, sand, and water predominated. The one class of crater is, strictly speaking, volcanic, the other hydro-thermal (if we give an extended meaning to this term), though undoubtedly the prime cause of action in both is the same. A glance at the map will show that the general character of the fissure changes immediately the base of Tarawera is left: it is no longer the narrow rock-bound rift like that on the mountain, but widens rapidly out into a large crater, or series of craters, through which, however, the narrower fissure can still be traced by the different craters marked on the map. This sudden change in its character is accompanied by a change in the nature of the rocks which have been blown out. The walls and sides of this large area are formed by soft earthy deposits which seem to be lacustrine in their origin, and the stratification of which is nearly horizontal, as may very plainly be seen in the steep cliff forming the eastern side. It naturally offered very much less resistance to the volcanic forces than that of the solid lava of the mountain, and consequently a very much greater amount of material has been removed.
The first point to which attention is drawn is the new lake, called after the smaller one which formerly occupied a position not far from it. This is an entirely new feature, directly due to the eruption. The lake is just one mile long, with an average width of nearly a quarter of a mile. Its eastern side is nearly perpendicular, but the western and higher one slopes much more gradually. In the beginning of August the waters showed signs of ebullition in several places, but very little steam arose therefrom. Its present height is 983ft. above sea-level, or 220ft. below the plain to the east. Plates Nos. 9 and 9A show the lake as seen from the northern end, and on the; eastern or left-hand side can be observed some horizontal lines, the middle one of which denotes the former surface of the country; the recent deposit (there mostly sand) being piled up above it from 30ft. to 50ft. A little stream of water, draining through from the Green Lake Crater, is found at the northern end, whilst at the south-cast side is another of considerable volume, which issues from the bottom of the cliff forming the side of the fissure. The water of the latter is warm, and of an astringent taste: where it leaves the cliff steam page 54 issues in small jets, accompanied by a disagreeable smell as of sulphuretted hydrogen.
The muddy-coloured water of the lake prevents any estimate being formed of the depth, but it is probably not great, and must be decreasing rapidly as the incoherent material on its banks gets washed down by the rains. A few days after the eruption a vast quantity of steam was observed to arise from this locality, and at that time no doubt the fissure was dry; but on the seventh day the steam had entirely ceased, as far, at least, as could be seen from the hills near the Wairoa, and the new lake had probably begun to form in the fissure. It was not, however, until about the 20th July, when Professor Thomas and Mr. Lundius passed round it, that its existence was known. No change had taken place since then up to the date of survey. We can only suppose the origin of this lake to be due to a cessation of activity in this part of the fissure at an early date, which would allow of the falling-in of the sides, aided by the heavy rains early in July, until the lower part of the fissure became completely filled up, thus allowing the waters to accumulate. There is no doubt that a large portion of the drainage of the south and eastern slopes of Tarawera finds its way by subterranean watercourses into it. That large bodies of water have come down from the mountain since the eruption is proved by the width of the watercourses on the plain, and by the great depth (over 100ft.) that a channel leading into the lake lias been excavated quite recently.
Plate No 9A
Drawn by G.N. Sturtevant from a photo: by C. Spencer.
Looking South-West Along Fissure
from Mt. Tarawera.
The fissure widens out somewhat suddenly at the south-west end of the lake into an undulating flat, covered with the usual grey sand and mud. The walls on the east are quite perpendicular and about 250ft. high, hut a talus of mud is found along the foot everywhere, and from it oozes out in several places a little cold water, which drains away into a lake shown on the map. On the opposite or western side the bounding hills slope upwards at a very steep inclination. They are indented by three crater-form hollows, from which a large amount of steam arises. The steam issues from a number of funnel-shaped depressions varying in diameter from a few feet up to 30ft. or 40ft. The two little lakelets shown in the line of fissure are both very active along their margins, though the water of the larger one is cold. The western side of the latter is formed by perpendicular rocks, which are cracked and fissured from top to bottom, from out of which the steam issues with great violence, giving rise to a deafening noise, to be heard a long distance off. The crater directly to the west of Star Hill, although small, is very active indeed, and is filled with steam up to its rim—so much so that no sight of the bottom can be obtained. The ridge which extends from the Banded Hill directly towards Star Hill is composed of earthy stratified clays ending abruptly against masses of hard rhyolitic rocks, which form its northern end, just above the lakelet before mentioned. These rocks are also steaming, whilst immediately at their base are several extinct fumaroles, some filled with water, others dry, and near them a strong odour of petroleum is perceptible. This ridge seems to have formed a division in this part of the fissure separating the Rotomakariri series of craters from those of Rotomahana; but in the direct line of fissure it is blown out, and the lakelet occupies that part as a separate crater, now nearly extinct, but which has been exceedingly active, judging from the large amount of rock-fragments scattered about. All over the place just described there are several funnel-shaped depressions marking the sites of fumaroles of various sizes now extinct. The general level of the plain is 980ft. above sea-level, or just 100ft. below the former level of Rotomahana Lake.
If we take the Banded Hill as a natural division of the fissure, it forms the eastern side of the great crater of Rotomahana, which has an east-and-west diameter from there of just one and a half miles, and a north-and-south one of about the same. This gives an area of page 56 slightly over two square miles as the size of the crater, which is surrounded by precipitous and in some places perpendicular walls, varying from 200ft. to 300ft. in depth. Below the walls the ground slopes by ridges and hollows to the central cavity, now filled by water. The height of this above sea-level is 565ft., or 515ft. below the former level of Rotomahana Lake. It will be seen from the map that the elongated lake or pool at the bottom of the crater occupies the line of the great fissure. The water is evidently hot, though none of the Survey party descended to it to ascertain the fact, the danger at that time being too great from the clouds of scalding steam which obscured a large portion of the crater. It is but a moderate estimate to assume that the depth of solid matter which has been removed from the crater by the eruption is 300ft.; but this will give a quantity equal to nearly 620 million cubic yards. In other words, it would cover two hundred square miles a yard in depth.
The Steam Cloud of Rotomahana from te Hape o Toroa hill.
(Looking North East) As seen in June 1886
From a photograph by C. Spencer.
* Appendix to Journal, House of Representatives, H.-26, 1886. The writer is glad to have this opportunity of correcting the only serious mistake which later explorations prove to have occurred in that report. It was there stated that only some doubtful specimens of fused rock had been met with: the fact is, the molten matter was covered up by the later ejection of sand, mud, and ashes, and it was only after the first rains that it appeared everywhere. Tarawera at that time was unapproachable.
Plate No 11
Drawn by G.N Sturtevant from a Photo by C.Spencer
Former Outlet to Rotomahana
now filled in with sand etc.
An attempt has been made in previous pages to show that, the origin of the eruption is deep-seated; and some evidence of this is demonstrable from the position the fissure here occupies with respect to the original valleys and hills. It will be seen from the map that from the south end of the Echo Lake Crater a valley, varying from 300ft. to 400ft. deep, runs, parallel and quite close to the fissure, right down to Rotomahana Crater—for a distance of over a mile. Now, had the eruption been in any sense superficial, this valley, offering the least line of resistance, and quite close to the general direction of the fissure, would be the line which it would naturally be expected to take. But, instead of this, we find that spur after spur has been severed, and the straight line of fissure continued quite irrespective of the formation of the country. Plate No. 13 illustrates this well. The crater there shown is blown out of one of the steep spurs coming down from Te Hape o Toroa, whilst the deep valley on the left, of the picture is left intact. Indeed, the explosive forces seem to have had, as it were, a preference for the spurs and hills rather than the lower intervening ground, for all of the five southern craters occupy the tops of the hills, excepting part of the Echo Lake Crater, which cuts right across a former valley.
From a photograph by J. C. Blythe.
From, foot of Hape o toroa hill looking South to Kakaramea, across "Black," "Echo," and Southern Graters.
Plate No 13
Crater North of Black Crater
shewing how fissure has cut through the spurs of Hapeo-Toroa Hill.
The Fissure from Top of Black Crater,
Looking Across Rotomahana to Tarawera in a North-East Direction.
The Black Crater.
From the West.
Illusrates pit-marked appearance of the hills where stone have been ejected and sunk in.
Echo Lake Oraler looking North East.
Shewing the large amount of ejected stone on the left; also the [unclear: center] in the side of hill called Gate of the [unclear: Inferno].
Reference was made on page 45 to the exception which the Black Crater offered to the general character of the southern part of the fissure. This exception consists in the fact of it and the Inferno having ejected molten rock of the same composition as that from Tarawera. The walls of the Inferno present the same appearance as the fissure on Tarawera—that is, the rock appears to have been subject to heat sufficient to partly fuse it. Several small volcanic bombs and a large number of rudely spherical masses of molten rock have been picked up in this locality, though nowhere else except on the mountains. These fusea globular masses vary in size from that of a walnut up to 4in. or 5in. in diameter; they are heavy and solid, and rarely scoriaceous.
The Echo Lake Crater, which lies immediately to the south of the Inferno, is divided from it by a low ridge about 50yd. wide. It is rather more than a quarter of a mile long, with an average width of 120yd. It has nearly perpendicular cliffs for most of its circumference, but at the north end these fall away to the level of the lake which now occupies the basin. In the beginning of August it had ceased to be very active, though the water was boiling here and there, and steam rose from the surface everywhere. Plates Nos. 15 and 16 give a good idea of its appearance as seen from the Southern Crater. The channel which leads away from the north-east end of this crater is now dry; but when Professor Thomas and Mr. Lundius passed along here in the middle of July, a large stream of very hot water was flowing from the crater-lake down towards Rotomahana. Where it leaves the crater the channel is about 20yd. wide, but it rapidly widens out to 120yd., and its bed is strewn with great rocks ejected from one of the adjacent craters. The rocks on the western side of the crater appear to be very solid below, and from fissures in them the steam still escapes. The mass of rock through which the crater has been blown out appears to be a hard, dense, banded sandstone of volcanic origin; in some specimens the bands or lamina; are highly coloured—green, red, and brown being the predominating tints. This crater (or craters, for there are probably two) has been page 62 the scene of explosions on a large scale, if we may judge from the immense number of rocks scattered around it, some of which are very large—as much, indeed, as a thousand cubic feet in solid contents. The floor of the valley to the east of it is strewn with ejected rocks in a remarkable manner: they are piled up in such a way as to lead to the belief that a continuous and heavy discharge took place in this particular direction for a considerable time; and, as they are not covered with the lighter ash and sand, they must be the result of the latest effort of the crater. Nowhere along the whole fissure are such numbers and varieties of unfused rocks to be found. The mineralogist will here find, over the space of six or eight acres, a complete museum of volcanic rocks. Although the east side is that on which the greatest number of rocks is found, all the hills and gullies for a considerable distance round are thickly dotted with them, either lying on the surface or buried under the cup-shaped depressions which characterize the hills. One of these hollows measured 30ft. in diameter by 12ft. deep, denoting where a rock larger than usual had sunk out of sight. The sand and ash on the edges of the crater is about from 15ft. to 20ft. in thickness. Plate No. 15 shows the old and new surfaces plainly.
To the south of the Echo Lake, and separated from it by a valley, is the Southern Crater, the dimensions of which are 200yd. long by 100yd. wide, and with a depth of 350ft. below the crateredge. On the fifth day after the eruption this was still very active, though filled with boiling water. An enormous column of steam arose from it high into the air, and obscured the surface of the water. The crater is blown out of a ridge, valleys dividing it, on the north from the Echo Lake Crater, and from the next ridge to the south. In neither of these valleys can the fissure be traced. It is somewhat remarkable that no sign of the fissure is to be observed south of the Southern Crater: not even a crack can be seen; but it is quite possible that the thick covering of ash hides any minor fissures or earthquake-cracks in this locality, whilst they are to be seen some distance south-westward. Plate No. 16c shows the nature of the country southwest of the Southern Crater, and in continuation of the line of fissure.
South West from Hape o Toroa.
Looking along country in the direction the great fissure would eoctend, if produced.
Shewing appearance of mud covered hills east of Rotomakariri, and the deeply furrowed surface due to rain water
"Rather within a mile to the north-east of the Puy de Chalar is a circular lake called La Gour de Tazana, about half a mile in diameter and from 30ft. to 10ft. deep. Its margin for a fourth of the circumference is flat, and little elevated above the valley into which the lake discharges itself. Everywhere else it is environed by a crescent of steep granitic rocks, rising about 200ft. from the level of the water, and thickly speckled with small scoria and puzzolana lava. These fragments are all that indicate the volcanic origin of this gulf-like basin; but these are enough. No stream of lava, or even blocks of any size, are perceivable. The encircling rocks show marks of considerable disturbance.page 64
* "Extinct Volcanoes of Central France." 1858. By G. Poulett Scrope, F.R.S., F.G.S.
"This curious and—in Auvergne—rather uncommon variety of crater is identical in character with some of the largest and most remarkable of volcanic maare in the Eiffel (particularly that of Meerfeld), with this only difference: that the former has been drilled by the volcanic explosions through granite, the latter through superficial strata of grauwacke, slate, and secondary sandstone.
"The peculiar character by which such craters are distinguished from the volcanic vents we have been describing—viz., their great widths, the total absence of all lava-currents, and the extremely small quantity of ejected scoria by which they are surrounded—are not easily accounted for. . . . . The extremely regular circular figure which is characteristic of all such craters demonstratively proves that the explosion of one or more clastic bubbles was concerned in their production."
"One remarkable and peculiar circumstance attends these cones of Montchal and Mont Seneire—viz., the existence of a deep, large, and nearly circular hollow immediately at the foot of each. The bottom is covered with water; and they bear the names of Lakes Pavin and Mont Seneire. Both are bordered with nearly perpendicular rocks of ancient basalt. Their position announces them to be contemporary with the eruptions of the neighbouring cones, and it seems probable that, like the Gour de Tazana already described, they owe their formation to some extremely rapid and violent explosion."
"In this range also, as in Mont Dome and Mont Dore, we meet with a few lakes, occupying wide, deep, and nearly circular basins, which bear every appearance of having resulted from some violent volcanic explosions, but differing from ordinary craters, not only in their larger dimensions, but in the nature, also, and disposition of their enclosures, which are usually of primary or, at all events, pre-existing rocks, merely sprinkled more or less copiously with scoria and puzzolana, little, if at all, elevated above the surface of the environing country."
No doubt Scrope is correct in assigning a sudden explosion as the cause of these and similar craters. After a few years, when the rains shall have washed the recent deposits off the hills, and the vegetation again appears near the southern craters, there will be little left to denote the origin of them but the blocks of unfused rocks which lie scattered around.