Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
Areas of the Hydrographic Basins in Canterbury and Westland, according to their extent
Areas of the Hydrographic Basins in Canterbury and Westland, according to their extent.
|Waitaki, of which 3140 square miles are in Canterbury, and 1774 square miles in Otago||4914|
|Hurunui, of which 538 square miles are in Canterbury, and 678 square miles in Nelson||1216|
|Molyneux or Clutha, northern sources in Canterbury||685|
|Taramakau, of which 26 in Nelson and 417 in Canterbury||443|
|Grrey and Arnold||184|
|Stafford, Cascade, Hope, &c.||428|
The river which is the most important and has the longest course, is the Waitaki, its length from the Tasman glacier, including Lake Pukaki, being 117 miles. Some of the West Coast rivers, although only 12 or 14 miles long, are nevertheless of considerable size, and notwithstanding they are flowing over a broad shingle-bed, they can only be crossed on foot after a continuation of fine weather. As pointed out in the first part, glaciers of considerable size are here situated near the termination of the outrunning spurs on the West Coast plains, and at altitudes of about 700 feet above the sea level, where pines and arborescent ferns are still growing most luxuriantly in the valleys and at the foot of the ranges. I think, therefore, that some observations on the occurrence of glaciers in such a low position and on the causes to which we must attribute such a remarkable phenomenon, may not be deemed superfluous. I may therefore be allowed to insert here a portion of a lecture delivered in Christchurch shortly after my return from the West Coast in 1865, treating on the subject,* and add a few more observations in further illustration. page 196Although we then possessed only very scanty meteorological material from the West coast, I may state that the observations since taken for a number of years at the meteorological station at Hokitika have amply verified my views on the subject, as stated in the lecture in question. From the time that the first explorer set his foot on the West Coast of this Island to the discovery of the goldfields, the difference of rainfall on the opposite coasts has always been a topic of great interest. It is obvious that the quantity of rain falling will seem still larger when the explorer is travelling in a forest which, generally, before it is thoroughly dried after a downpour, is again soaked through by new showers.
When writing in 1865 on the subject, we had not sufficient data to go upon to determine, with accuracy, the differences, in inches, of the rainfall between the two coasts, although the valuable observations of Dr. Hector, in 1863, for seven months, from the first of June to end of December, showed that there fell, in the south-western part of this island, 87 inches, whilst in Dunedin it was only 23¼, proving that the quantity of rain was more than three times and a-half as great at the West Coast as at the East Coast. Concerning the difference between Christchurch and Hokitika, we had only reliable data since the 29th of April, 1865, when Mr. John Rochfort set up a rain-gauge at the latter town. From the 29th of April to the 3rd of July, 1865, inclusive, 67 days, 36½ inches of rain fell in Hokitika, whilst in the corresponding period it was only 7½ inches in Christchurch; consequently, about the fifth part. In the meteorological reports of the colony for the year 1875, the annual rainfall for 1874 is given for Hokitika as 104·480, and the mean for the previous eight years as 113·116; for Christchurch, as 22·790, and for the previous ten years as 25·727; consequently, although in some years the annual rainfall will be more than five times as much at Hokitika than at Christchurch, the average will reach to about four times and a-half.
As I observed already in former publications we have to seek the cause of that enormous difference in the position of the West Coast, so well exposed to the equatorial currents, which bring with them a greater amount of rain everywhere, where the same conditions exist, and of which I shall give only a few instances. The rainfall at the north-west coast of America is 80 inches; at Bergen in Norway, 83 inches; at Coimbra, in Portugal, 110 inches; and at Westmoreland, in England, as much as 134 inches annually. That there is also such a similar heavy rainfall at the western coast of South America we know page 197from Darwin's classical works on that region. Like our own West Coast, the former is covered with a dense and uniform forest vegetation, which, of course, again favours the condensation of the clouds, and, consequently, the fall of rain; but these dense forests are generally not the cause of the rainfall, as popularly has been assumed, bat are a consequence of it. It is obvious, from the fact of the snow-line, which owing to the equable and humid climate on the West Coast, is very low, probably about 6000 feet near Mount Cook, and from the fact that the fall of snow and condensation of moisture must be still greater in those higher regions, where equatorial currents come in contact with the cold surfaces of the Alps, that all necessary conditions exist not only for the formation of large glaciers, but also for their descent to much lower regions than at the east coast.
Standing at the sea-coast near Hokitika, I very often observed that the mountains bounding the West Coast plains were covered with nimbus or rain clouds, whilst we enjoyed fine weather near the sea. At the same time, very often, smaller freshets in the rivers could be observed, when not a drop of rain had fallen near the sea-beach, all confirming the still larger amount of moisture falling in the higher regions. The difference between the eastern and western side of the central chain is well exhibited by the great Tasman glacier, which, although of much larger dimensions than the Francis Joseph glacier, yet descends only to 2456 feet above the sea-level, whilst the latter reaches more than 1700 feet lower, namely, to 705 feet above the sea. It is true that particular circumstances—as, for instance, a large cauldron-like basin, sheltered from the sun's rays by Mount de la Beche and its outrunning spurs, in which these enormous snow masses can accumulate, is very favourable for allowing that glacier to descend to such a low positon above the sea-level, where arborescent ferns, pines, and other low land trees are growing. But if we compare its position with others in South America, we shall find that, from ranges which are not so elevated as our Southern Alps, even in latitudes corresponding with the northern end of Stewart's Island, enormous glaciers descend in latitude 46 deg. 50 min., according to Darwin, to the level of the sea, their terminal face being ultimately washed away and carried along as huge icebergs. Thus the conditions for the lowering of the snow-line and of the excess of moisture must still be greater in that part of America than in New Zealand, where the neighbourhood of Australia and Tasmania will certainly exercise some moderating influence, which in Terra del Fuego does not exist. From observations made in those page 198and other regions, it is clear that the lowering of the snow line does not depend on the mean temperature of the year, but on the low temperature of the summer. The mean summer temperature in Christchurch in 1874 was 62 deg., and in Hokitika 58·9 deg., the difference in favour of the east coast being without doubt attributable to the clear and cloudless sky we so often enjoy during summer. Thus, the overcast atmosphere, in combination with the far greater rainfall, accounts for such a lowering of the snow-line on the western side of the Alps, when compared with the eastern slopes.
However, this deficiency of the Hokitika summer temperature is more than compensated in the winter, when, in Hokitika, in 1874, it was registered as 45·8 against 42·6 in Christchurch, the difference being 3·2 degrees; this fact thus fully confirming my previous opinion as given in 1865, when no corresponding observations had as yet been made at the West Coast. I may here add that the annual mean temperature was 52·6 in Christchurch, and 53·8 in Hokitika, or 1·2 higher at the West Coast during that year. The position of the Francis Joseph glacier is about 43 deg. 35 min., corresponding in the Northern Hemisphere with that of Montpelier, Pau, and Marseilles in France, and Leghorn in Italy, where the orange and lemon tree, the vine and the fig tree, are covered with juicy fruits, and where palm trees raise their graceful crown into the balmy air. Even in the European Alps, which lie some degrees further north, the average altitude of the terminal face of the larger glaciers is about 4000 feet whilst we have to go twenty degrees more to the north, till we find in Norway, glaciers descending to the same low position as the glacier under consideration, and to about 67 deg. north, according to Leopold von Buch, before the terminal face reaches the sea; consequently more than 20 deg. more towards the Pole than in the Southern Hemisphere, in Terra del Fuego.
All the principal meteorological phenomena encountered in the European Alps, and which have been described and explained so differently, according to the point of view taken by each writer individually, also occur here, the nor'-wester of New Zealand (equatorial current) being simply the föhn of Switzerland or sirocco of Italy. As formerly pointed out, the snow-fields and glaciers of the Southern Alps when compared with those of Europe, are of much larger dimensions, especially if we take the altitude of the mountains into due consideration. That they were formerly of still more gigantic proportions is, amongst other indications, well shown by the line of page 199lakes on both sides of the Southern Alps, and the enormous moraines surrounding them, which mark clearly the latest extension of the postpliocene glaciers.
Discussions such as those which were going on some years ago between Professor Dove, of Berlin, and some of the principal scientific men of Switzerland, would have been much simplified had those gentlemen been acquainted with all the characteristic features of our nor'westers, being in every respect identical with the phenomena described by Professor Dove, with whose writings I am best acquainted, and with whose conclusions I entirely concur, as being the characteristics of the föhn. In fact, his description of the föhn from its first setting in on the Italian side of the European Alps, its crossing and effects on the Swiss side is such, that if we change the word Italian for western, and Swiss for eastern side, every inhabitant of this island who has travelled across it would consider it a faithful description of our nor'westers as travelling from coast to coast. However, I may point out that occasionally our nor'westers do not bring rain with them when crossing the height of land, having descended before they reached the Southern Alps, thus becoming deprived of the principal portion of their moisture on the sea or on the low lands lying at the western foot of the ranges. When these winds pass across the snow-fields of the Southern Alps, the cumulus clouds creeping up, disappear as by enchantment, and the sky remains of a deep blue colour, but the wind sweeping down the valleys is very bot, and the rising of the glacier torrents shows at once its effect. A theory tracing them to the interior of Australia would be difficult to prove.