The Pamphlet Collection of Sir Robert Stout: Volume 33
The Geological Action of Fire.*
The Geological Action of Fire.*
The subject selected for this evening is the geological action of fire. It is particularly desirable to have a clear knowledge of this powerful geological agent as preliminary to almost all geological study. You will find that the most simple and elementary parts of geological research require some knowledge, and tolerably exact knowledge too, of the way in which fire acts in its geological relations both to the structures of the great rock-masses, and the general conformation of all the older rocks—from which those less old have been subsequently formed, by various mechanical and chemical changes.
I have placed a number of rock specimens on the table, which furnish good examples of all the more ordinary "Igneous Rocks," as they are termed, and they show the comparatively few mineral ingredients of which all the rocks manifestly formed by the agency of fire are composed. If you examine these carefully when you have more leisure and better light, in the day-time, you will find such study by the eye greatly to facilitate your advance in geological knowledge, particularly of that portion of it called "Lithology," which may be taken in any point of view as a necessarily preliminary portion of all systematic practical geological investigation.page 2
The most important study as to the direct action of fire, in a geological point of view, may be made in the various localities of the earth affording examples of what are commonly called "active" volcanoes, such as Vesuvius or Etna.
In all eruptions of active volcanoes, you have in distinct action this great geological agent, actually forming before your eyes a number of rock-masses of almost all the varieties of the newer kinds of igneous rocks, under circumstances and peculiar local conditions which are seen to be capable of producing great differences in the general appearance of the different recognised kinds from a nearly uniform material; a fact readily to be understood when you see these causes producing the effects during an ordinary volcanic eruption.
In existing volcanoes you find the most universal characteristic is the outpouring of great rock-masses in a melting condition, or of pulverulent, dry, dusty, or cindery masses mineralogically identical with them, from the volcanic craters. It is a matter of great delight to the geologist to find that the melted rocks which he sees thus flowing out from the interior of the earth, and cooling and solidifying almost under his foot—that these rock-masses forming under his eyes are identical with those he finds in the earlier periods of the world's history, forming great thicknesses of the earth's crust; so alike, in fact, that it is in many cases almost impossible to tell one from the other.
On looking at any ordinary volcano, as, for instance, Vesuvius, which we have sketched here on the black-board, you commonly see the appearance here represented and described by old Italian geologists as resembling a pine-tree, that sort of pine-tree you see in Turner's Italian Landscapes, being apparently a narrow cylindrical column of smoke rising from the aperture at the top of the mountain to a great height, and then spreading out like an Italian pine, or an umbrella, or mushroom. This is not really smoke, but a mass of pulverulent fine dusty particles of the very common minerals which compose the great flows of melted "lava," as the boiling rock which pours down from the craters is called.
These dust-like particles, composed chiefly of minute crystals of igneous minerals, get into the higher regions of the atmosphere, and are carried by currents of air sometimes hundreds of miles over the land or out to sea, where, as they gradually fall, they constitute rock masses which were excessively puzzling to the older geologists, having the mineral and physical constitution of igneous rocks to the eye, but often page 3 containing the remains of shells, or bones of quadrupeds, if on land; or fishes, corals, &c., similarly entombed at the bottom of the sea; whereas such remains would be utterly destroyed by a heat far less than that necessary for the production of the material composing the rock-mass. These submarine volcanic rocks are, however, so little popularly known that I need not dwell on them further than to point out that geologists recognise them as due to volcanic action, and that these volcanic ashes are found interlaminated in great beds through Snowdon and other large mountains in North Wales; for instance, in Russia, in North America, and in Canada, &c.; the term trap-ash being often applied by modern geologists to such igneous rocks deposited from suspension in water or from the air on land.
There is another igneous rock formed in nearly the same way, to which Italian geologists give the name of "peperino" and this is abundantly distributed over the country in the neighbourhood of active volcanoes. When an eruption takes place it is commonly accompanied by violent thunderstorms and heavy rains. Now, when we consider that the clouds of apparent smoke issuing from the craters are in reality great clouds of fine dusty materials, you can readily conceive that the raindrops falling through such dust would be coated over like pills, and come down somewhat in the shape and size of peas, forming a peculiar mass over the surface of the earth, to which, as I have said, the name of peperino is given, and having a strange analogy in many respects to the volcanic deposits occurring in the bed of the sea; inasmuch, for example, as this peperino, and the ashes brought down with it through the air, will very often appear stratified as if it had fallen through the water, and cover over field-mice and other small quadrupeds, birds, reptiles, &c., on the land, and other living creatures in small accumulations of fresh water on the surface, which in this way become embedded on the land in great quantities, although they certainly would have been utterly destroyed if the rock enclosing them had been at the time in a condition of igneous fusion.
The same thing pretty nearly occurred at Herculaneum and Pompeii, where a great overflow and outburst of volcanic mud, mingled with a large quantity of water as it came from the mouth of the volcano itself in the case of the one city, and showers of hot ashes in the other, entombed works of art and human bodies so rapidly, and preserved them so completely intact, from the first century of the Christian era down to our page 4 own times, that we can study their social peculiarities almost as perfectly as if they were of yesterday.
I mention these two kinds of igneous rocky material in the first instance because they are so unlike all the other and commoner kinds, although related in composition; and having so spoken of them, we may now consider the commoner kinds by themselves. In nearly all existing volcanoes, the commonest rock which is thrown out resembles the lump now in my hand, looking very much like a clinker from the furnace, or a large cinder. It is to this vesicular kind of stone that the term "lava" is generally applied. It is generally composed of two minerals—one, the commonest of all minerals in igneous rocks, felspar; the other, the black mineral augite, of which there are several specimens now lying before you. These two mineral materials may be readily melted in a blacksmith's forge at the heat melting ordinary pig-iron.
If this melted rock happens to flow over the surface of a level country, it almost invariably presents this appearance like petrified sponge, due to the great number of little holes in the stone caused by the air, steam, and other elastic vapours of various kinds given out with the melting stone, and expanding according to the shallowness of the quantity of stone flowing over the surface, forming large bubble-like holes when the pressure is small, but becoming small or imperceptible as the pressure increases with a greater thickness. But here before you is a much heavier stone, and perfectly compact, called "basalt," very unlike the vesicular lava in appearance, but which you would, on minute examination, find really identical in mineral composition. If the lava, instead of flowing over a flat plain, happened to get into a confined space, such as a narrow valley or river-course, where to a considerable depth it might be jammed together, you would find that in the lower part of such a mass the holes were not to be found, but that the stone became one of the most compact, toughest, and heaviest of rock masses with which we are acquainted—basalt; and at greater depths, producing greater pressure and slower cooling, the separate minerals would crystallise in separately recognisable portions, forming greenstone; the dense rock and the crystalline one and the vesicular lava being identical in composition; the differences being caused by pressure and the rate of cooling.
There are other great extremes due to exactly the same causes. If the cooling be much more rapid than that commonly producing lava, you get the bimstein of the German page 5 lithologists, the pumice-stone used by painters, excessively light, often floating in water, but still perfectly identical with the heavy rock masses with which we have been dealing, the only difference being the greater quantity of air cavities in it, surrounded by thin envelopes of rocky matter, caused by such a mass being thrown up rapidly in the air to a great height, and the consequent rapid expansion of the gases contained therein. Another extraordinary difference may be produced in the same way. I have in my hand what looks like a piece of common bottle-glass—ordinarily called "obsidian," or "volcanic glass"—of a dark greenish black colour, translucent on the edges, and with a conchoidal fracture, and so nearly resembling, as I said, black bottle-glass, that in Wales and some parts of England a very good trade has arisen from making bottles by melting a stone like what the Melbourne builders call blue-stone, and cooling it suddenly so as to form this material. Yet it is perfectly the same in its composition as the vesicular lava, or pumice-stone, or basalt, or greenstone, which I have brought to show you, merely differing in the fact of having been still more suddenly cooled. Our bluestone is only another form of this rock-mass, having the same components—felspar and augite—and from which you pass insensibly to the hard black basalt, or if the cooling has been still slower, to another change, as in this stone I now show you, to which the name of "greenstone" is often applied, and where the constituent minerals separate themselves, the felspar forming little crystals by itself, and the augite or hornblende another series of crystals by itself; and in the different "bluestone" quarries about the town—as at Richmond, for instance—you may see all these varieties in unbroken masses of stone, some vesicular, some crystalline, and some dense and forming prismatic basaltic columns, like those of Staffa or the Giants' Causeway, on a small scale, and occasionally the glassy form.
If the cooling be moderately slow, it gives rise, under certain conditions in the same rock-material, to another set of rocks, of which porphyry is the type, in which some one of the materials forms large crystals, imbedded in a general mass composed of smaller crystals, or of a massive compact character, the material of the crystals being generally the felspar.
(The lecturer here exhibited and explained a great variety of rocks in detail.)
The general fact, then, established by investigation of volcanoes is, that if all these materials were mixed together and page 6 united in the interior of the earth, the many very different-looking igneous rocks would be formed, merely as the results of the different rates of cooling of one and the same stony mass. In all the older geological times the same variety of material has been certainly produced by the same cause—that is to say, by the rate of cooling from the state of fusion due to the intense interior heat; but besides this you find that some extraordinary generalisations present themselves, of great interest to the geologist, from the constitution of the igneous rocks formed at different periods of the earth's history. As a general rule, the very oldest igneous rocks present a considerable portion of the mineral quartz, but this quartz is excessively rare, or not present at all, in the more recent volcanic rocks flowing from active volcanoes.
Another mineral in the ancient igneous rocks is mica, which, like quartz, is also as a general rule very rare in the more recent ones. There are several ways of accounting for the different mineral composition of the igneous rocks of different ages, but let it be here borne in mind that the general subject of geological age cannot be measured by years or historic periods, but by relative periods; and the various geological epochs are generally indicated by reference to the various formations of stratified or sedimentary rocks succeeding each other in the order in which they were deposited in the sea of old time; it being obvious that a lower bed of rock if so deposited in the water must be older than another above it, and so on, each of these groups of beds having a name attached to it, as the silurian, the oolitic, the chalk, &c., represents the period of time occupied in its formation; and although people may differ as to the number of years any geological formation may have taken, there is no difference of opinion as to the relative ages referred to by the names of the formations.
The great series of geological beds of rock representing geological times are found as a general rule to rest on a basis of igneous rock, frequently that which is represented in a diagram before you as lying underneath all the bed-rocks, and to which the name of "granite" is given, in which the scaly mineral mica is excessively abundant, and the mineral quartz is also more abundant than in any other igneous rock whatever; these two, with felspar, are the principal components of granite.
Now, quartz and mica require an extremely intense heat to allow of their fusion and crystallisation, greater in fact than page 7 we can produce artificially, although of course they were thus originally fused.
According to the theory of Professor Abich, the materials of all these rocks which you see here depicted on these diagrams before you, were originally fused in one great melting mass; and when that mass in cooling first began to solidify, you can conceive that if all the mica, quartz, and felspar were in that seething mass, the first thing to solidify would be that which required the greatest heat to melt it—the quartz, then the mica, would solidify or crystallise first, as they are much less fusible than the others; and thus, from this infusibility of the two principal materials you would necessarily have granite as the first rock product of the cooling earth, with nearly all the quartz and nearly all the mica of the general mass, mixed with felspar, which although very fusible would probably be entangled with the others. This would explain why quartz and mica abound more in the ancient igneous rocks than in those of more recent formation resulting from the cooling of the remainder of the constituents. There is an interesting point to bear in mind here, that you should count the age of such igneous rocks in the opposite direction to that of those deposited on the bottom of the sea; the bedded rocks, as they are called, naturally becoming newer as you .approach the surface; whereas it is obvious, according to the theory we are considering, that under a crust of granite first formed you would then have the materials for another crust, say of greenstone, in which there would be no quartz, but a great deal of felspar and hornblende; then, probably, still lower a series of porphyries and trachytes, with still more felspar, then basalt, and so on.
Professor Abich was the first to draw attention to the fact that the most modern of these rocks are excessively dense as compared with the older igneous deposits, and he points out that, after the granite had abstracted all the quartz and mica, leaving the ordinary constituents of the trap rocks, there would be a tendency in the melted rocks below to range them selves according to gravity, the heavier ones having a tendency to sink to the bottom, thus explaining the fact of the basalt rocks coming up to the surface of the earth after syenites, greenstones, and other nearly allied kinds, which we find more abundant in the older and middle periods of geology.
I have dwelt longer than I meant on this part of the subject, but it was necessary to do so before proceeding to speak, as I now will, on the characteristics of volcanoes.page 8
When you examine existing volcanoes, you find that, although situate in very different localities, they differ but very little from any one type you may take as an example, and so nearly all the old instances of outbursts from below or within the earth of molten rocks would be found to have followed almost exactly the parallel characteristics of those of the present day.
Suppose, then, we consider the character of Mount Vesuvius, as here depicted in this long drawing, and which furnishes a very good example of the characteristic form of nearly all ordinary volcanic vents or craters, as generally formed.
Of the ancient crater of Vesuvius, the portion remaining, and called Monte Somma, is about 4000 feet high, having an irregular, broken, sharpedged outline. The earlier physical character of this was a great circular valley, as you see here drawn, with a sharp serrated edge higher on one side than the other, but very precipitous, highly inclined on the outside and almost perpendicular inwardly. Below this extended a nearly level plain, on which a few flocks were fed; but there was no record of the igneous character of the mountain, nor even the slightest idea of its real nature, until the first century of the Christian era—not even a tradition of activity of any kind, until the occurrence of a very violent earthquake, which damaged several neighbouring cities; and then, after a succession of alarming shakings, a final one occurred in the month of August, A.D. 79, accompanied by a sudden explosion and bursting out of the whole of the central part of the valley, giving vent at once to an enormous shower of red-hot ashes, cinders, and scoriœ, as also of great streams of volcanic mud, entirely overwhelming the great cities of Pompeii and Herculaneum, the remains of which travellers can now examine at their leisure, the eruption preserving, at the same time that it buried them so completely as to form a level plain above the tops of their highest buildings.
From that time to the present the volcanoes of Italy have remained active. But the portion of Vesuvius now generally chosen for volcanic study is a small crater called Monte Nuovo, which arose in the year 1538 to a height of about 430 feet from the base of the original Monte Somma, and having a circumference of 8000 feet. It rose in about two days, and forms an inner cone in the centre of the great original one. I wish particularly to draw your attention to a general character of almost all volcanic craters in the world, represented in the drawings before you. You see the sides coming up with a steep slope, very nearly at that particular angle page 9 which is known as "the angle of rest," an angle you can imitate on the sea-shore or elsewhere by letting sand fall from your hand till it forms a small heap, the term being applied to the invariable degree of inclination that sand or other similar material would assume under such circumstances—the slope at which the grains would have no tendency to roll down to the lower part of the heap. Recognising this particular slope is of great importance in the recognition of the existence of volcanoes in various parts of the world, and is intimately connected with the formation of these volcanic cones.
When a section is made through a volcanic cone there is an appearance, as you see in the diagram, of beds sloping at the same angle or nearly so as the outside of the mountain, which beds are composed of layers of peperino, pumice stone, vesicular basalt, and other stony material, forming a series of deposits apparently sloping in every direction from the centre.
Baron Von Buch, in an elaborate treatise written by him on craters and their elevation, supposed these appearances to be indications of a series of horizontal beds thrust up in the centre by some great force acting from below that centre, and his term "crater of elevation" indicates his theory of the formation of volcanic craters by such influence.
But the real fact is, if you consider for a moment such an opening as this, in the middle of the plain, continually throwing up stones and ashes for ages, such ejected materials falling, as I instanced just now, like sand from the hand, the result must be a conical hill, with precisely the angle you find here, viz., the angle of rest. When you can get a cause in this way capable of producing a given effect, it is a safe plan to refer such effect to such cause, instead of to one of a less applicable kind.
When these cones are formed in this way, the occasional overflowing, during eruption, of the melting lavas rising in the crater, and flowing over in sheets from the top, will give alternate thicknesses of different kinds of rock, producing the different layers of stone, ashes, &c., observed; and all the best geologists are now agreed that Von Buch was in error in applying his theory of elevation to most craters.
Having drawn your attention to the peculiar angle of the side of a volcanic crater, I will now point out another characteristic of form of all existing active volcanoes.
Having mentioned that Monte Somma was much higher on one side than the other, so I may inform you that Monte Nuovo presents exactly the same appearance, the top of the page 10 crater being obliquely truncated and the lower side broken through, and thereon comes the explanation of why one side is lower than other. You will find, when you consider it, that in such a cone as this, with melted rock below seeking a vent, forced up through this long pipe (as it appears in the diagram) towards the summit, every addition to the height of the column enormously increases its bursting power, and when at last it becomes of such a height and power that it can be contained no longer, it bursts out on some one side or the other (usually that towards the general fall of the country); and whichever side it is that gives way, there is a great fracture inside, through which the current of melted rock is run out to the lower levels. You may recognise in many countries of the world, by these peculiarities of form, many hills of the nature of true volcanic cones, notwithstanding that they have long ceased to be active. There are in this colony many of them of a most perfect kind—the same outside slope, precipitous near the edge, and an oblique truncation of the hollow top of the cone (sometimes containing a small lake), and in the broken lower side you find traces of the overflowing current of melted rock which has produced that peculiar form. A beautifully perfect one, just like some of the extinct volcanoes of the Rhine or Auvergne, may be seen at "Jim Crow," or Mount Franklin, near the native station.
Studies of this kind led a number of French geologists, followed by Mr. Paulet Scrope, the famous English one, to the discovery of a great number of extinct volcanoes in Auvergne. In Central France, and Auvergne particularly, you find such appearances as these conical hills, with just this angle of rest, and formed of scoriœ, ashes, &c., and rising to a sharp obliquely truncated hollow upper part; and from the lower portion of the top edge a great stream of basalt, 20 or 30 miles in length, usually found running down to some low level. You can see the same sort of thing at Mount Franklin, or Jim Crow (as it used to be called), in this colony. You also see hero the places in the broken lower summit from which the last flow of bluestone took place, with the flow marked by its having cleared its way apparently through the forest, and making its way with gum-tree forests on either side; the lava stream winding before you as far as eye can reach, but without a trace of vegetation on its course, and looking as fresh as if it was flowing yesterday, but which probably occurred in the later tertiary geological period of the old time.page 11
Whenever the stream of lava, or a portion of it, gets into some cavity where it can cool more slowly, you often find the curious result of basaltic columns, of polygonal jointed prisms, as effect following the cause of such retarded cooling; as, for instance, in these illustrations before you of the celebrated Giants' Causeway and Fingal's Cave, in which you find a stone nearly identical to that (bluestone) which you find in many places going from Richmond to Ballarat in this country.
A curious experiment, illustrating this columnar formation on a small scale, has been made in England by Mr. Gregory Watt, and since repeated by others, by taking about seven hundredweight of common bluestone, there known as Derbyshire toadstone, and keeping it liquified in an iron-smelting furnace for eight days, then allowing it to cool slowly. He found the outside to be composed entirely of volcanic glass; then inside a mass resembling the rock to which the term pitch-stone is applied; then jasper; then a material nearly resembling basalt, without particular form; and then he found, a little within that, in the still more slowly cooling basalt, a tendency towards the formation of concretionary masses, producing, farther towards the centre, from their greater size, number, and mutual pressure, a series of prismatic columns, radiating from the central part towards the periphery, and jointed exactly as you see here represented in these delineations of the Giants' Causeway and Fingal's Cave. Dr. M'Culloch, in his work on the rocks of Scotland, pointed out that in some localities where these columnar jointed masses of basalt rotted or decomposed, the first parts to give way were the corners at the joints of the columns, this giving rise to an appearance, recognised by many writers, of a hollowed surface to some of the joints, with a convex rounded surface on others, as if they fitted into each other like cup-and-ball joints. This very acute observer pointed out, that if the corners came off from the joints, and this process continued, the inevitable result at last would be that the whole of the angularity would be lost, and you would have, in fact, a series of spherical, concretionary masses, just like those produced by the experiment we have referred to; but, originally, concentric surfaces, compacted into this appearance merely by lateral pressure. The experiment clearly showed how this is produced, and you may trace the same action in some of the moving streams from Vesuvius at this day, stone assuming that prismatic character generally at right angles to the plane of cooling.
Several most important characteristics of igneous action may page 12 be found in Mount Etna. In the diagram before us we have a portion of the interior of the great cone of Etna, with a section of a lesser cone, showing an appearance of stratification, which is the result of successive deposits of lava and ashes; and at various places you see great basalt buttresses standing out through fissures in the sides of the mountain, forming portions of what are solid walls of stone, nearly vertical, and traversing the scoriaceous beds in various directions, and terminating above in great sheets extending obliquely downwards over the side at the height it had attained at that time. These are known to geologists by the name of "dykes"—great vertical veins of igneous rocks, apparently cutting through the geological formation of various portions of the earth, but really intrusions of melted rock, usually from below, filling up great cracks or fissures in the earth, but sometimes cutting those channels to flow in. Nothing is more common in the coalfields of England than to find a basaltic or "trap dyke" thus cutting apparently through the coal-beds, and showing the geological action of fire on those beds in the same way that the same coal would show the action of fire in the retort of the gas works—namely, by the distillation out of it of all the bituminous or gaseous constituents of the coal from near the dyke, where it is converted into cinders or coke; then, as you gradually recede from the dyke, you find that the mass of cinders, or clinkers, into which the coal is converted, becomes gradually changed into what is known as stone-coal, or anthracite, a kind of coal useful for many purposes, but remarkable for burning without flame, having no bituminous matter to produce it; and still farther off the ordinary character of bituminous coal is found unchanged. Similarly, dykes are often found traversing beds of dark, fossiliferous limestones, and changing them into snow-white, statuary marble, with a crystalline fracture like loaf-sugar, and destitute of petrifactions—changing them in the same way as great heat and pressure have been found to change such limestones experimentally. Thus, then, you find these appearances in Etna perfectly similar to those we know of in the oldest formations in the depths of the earth.
I now wish to draw your attention to a modification of the volcanic crater in the Sandwich Islands, as depicted here in the drawings before you, interesting from the difference presented from the ordinary forms I have already dwelt on.
It is obvious that for the production of active volcanoes you must have a large quantity of melted rock below the earth's page 13 surface, forcing up through craters, and spreading out in the different forms of rock I have briefly indicated to you; and in all volcanic and earthquake regions you must necessarily have this great molten subterranean supply. But very few of us could expect to see such a fearful sight as Dana made known in his description of the extremely different craters in the Sandwich Islands from those generally known.
In this drawing of the one of Kilauea you see a lateral cone at a much lower level than the great cone of the island to which it belongs, and acting as a sort of safety-valve. In the crater of Kilauea you find an enormous flat-edged opening in the crust of the earth (showing how very thin or insecure that crust on which we walk is), which seems merely an egg-shell-like covering over this vast area of seething fire below. In the middle of a great plain you see (as here depicted) a great hole, which is in reality several miles wide and long; looking down which you see the very unpleasant artistic prospect below of a lake of glowing, boiling rock, now rising to the brink so as almost to overflow, now sinking to a great depth. To give you some idea of the extent of this hole, it is suggested that if the whole city of New York were put down there it would be scarcely discerned. The whole of this is one vast boiling cauldron of seething rock, sometimes uprising near the edge, sometimes sinking for many feet, spurting out volumes of sulphur, steam, and other gaseous ebullitions; looking by day-light something like the pitch lake of Trinidad, but at night glowing of a dull red heat as far as the eye can reach.
The envelope of all this is almost a plain, with none of the acute angular, elevated, conical character of the ordinary craters; but there you see one of the very few instances that can be observed of a simple thin crust of earth, floating itself, resting insecurely on this enormous seething, melting mass of rock below, and which underlies so much of the earth on which we are dwelling in fancied security. Professor Dana describes how, while sitting on one spot in this plain, he saw the very place he had sat upon the night before crumbled in, melting immediately, and forming a portion of the fiery mass rising to the edge and then again subsiding.
This is, then, one of the very rare cases of a lateral vent connected with one about two and a half miles high, or more than twice the size of Etna, and which, with all the characteristics of the ordinary crater, occasionally gives outbursts of frightful volcanic violence, but seems to be continually relieved by the gentle boiling up and down below. It is page 14 certain that when this boiling flood rises to a certain height it flows out laterally under the sea, and thus gives enormous relief to the seething mass within.
But time warns me that I am unable to dwell longer on the direct application to geological subjects of the action of fire, although the subject is but half-touched; and I must, therefore, now, with thanks for your attention, conclude for the evening.
* This lecture is printed from the reporter's notes, as revised by Professor M'Coy.