WCP4864

9, St. Mark’s Crescent N.W.

April 4th. 1869.

Dear Sir Charles

I will first make a few remarks on your letter on Glaciers.¹

1. Whatever time the Glacial epoch lasted in Scandinavia[,] Scotland or N. America, that will be very far in excess of the time that it can have lasted S. of the Alps, where some of the most remarkable lakes occur. In all probability the extreme cold came on there much later & departed earlier than in more northern countries. On the theory of the Italian lakes having been caused by upheavals or depressions which were preserved by ice, we must assume an exceptionally rapid rate of motion in that district, or else we ought everywhere to find proofs of such rapid motions either in filled up lake basins, or in tilted up valleys through which the river has cut an outlet. We ought to find [2] many valleys with such a section as this [Diagram of a valley with a river channel with the labels ‘bottom of valley’ and River channel']. Are there any such valleys in which the broad flat valley bottom itself rises up against the average downward slope of the valley, while the river flows in a narrow gorge? Yet if valleys have been tilted up often enough and rapidly enough to form deep lakes wherever the hollows were preserved by ice, surely such hollows in the valley bottom itself ought to remain ^{in places} where, owing to the absence of ice the river cuts it's way out by a gorge as fast as the land rose.

__________

2. There can be no doubt that subsidences during earthquakes have produced many lakes:— the question is solely the number, form, depth and distribution of those lakes; and [3] these seem to me incompatible with any such general explanation of the lakes in glaciated countries and those in the much larger surface of the rest of the globe.

__________

3. It would not be difficult to measure with some accuracy the quantity of matter suspended in the water which issues from a glacier. If this was done for several large glaciers, throughout the year, we should have a measure of the grinding power of ice over given surfaces;— & by a careful survey of the glacier itself, the thickness of the ice might also be pretty nearly arrived at. This would be a most worthy object for a grant from the Royal & Geographical Societies.²

4. I maintain that the true glacier-lake-theory, is neither scooping not pushing,— but simply grinding rocks [4] to mud, which is carried away in suspension by water, & would therefore escape just as well out of a deep lake as out of a shallow one, as there must be enormous hydrostatic pressure under the glacier in a lake basin forcing up the for water with great violence. These are real and known causes, — the scooping and pushing are to a great extent hypothetical.

5. As far as I am concerned, I would give up the Great Lakes of N. America as being due to ice-action alone; till the question of the Swiss, Italian, & British Lakes is settled. In the former case we know nothing of the thickness of the ice-sheet. In the latter we have clear evidence of ice filling the valleys thousands of feet deep. Let us settle the easier before we go to the more difficult problem.[5]³

6. The absence of lakes in the Himalayas is due probably to the enormous erosive power of the torrents. There have been lakes. In Godwin-Austin’s (Capt.H.H.) paper on the Glaciers of the Mustakh Range (Trans[action]. Roy[al]. Geog[raphical].Soc[iety]. vol. XXXIV. (1864) p.23.)⁴ he describes lacustrine deposits 4000 ft. above present valley of Iskardo, which has evidently been a curved lake 40 miles long, and 4000 ft. deep! ^{This lake} which has some been exceptional by been filled up by the torrents, and subsequently cleared out again by the barrier being cut down. It is situated just where you would expect a glacier lake. There ^{is the} meeting of great valleys in two or three directions accumulating glaciers from an enormous range of mountains, an abrupt doubling back of the valley, & a barrier below through which a gorge is now cut ^{and the blocking up of wh[ich]. still occasionally gives rise to floods.} He gives an interesting map. [6]

7. As to Fuegia [Tierra del Fuego] & Patagonia. I should say that Magellan’s Straits with their immense bogs & inlets were once lakes which have been united by subsidence. Other large fjords & inlets in ^West Patagonia seem to have been produced from lakes in the same way. An accurate survey of the bottom of Magellan’s Straits would show this but I can hardly conceive of those straits being produced in any other way.

8. It is of course hopeless to attempt to explain all difficulties, especially in cases where our knowledge is confessedly imperfect.

I do not see however how you are to get over the objection in my 1st par[agraph]. except by showing that in warm countries there are numerous valleys which in some part of their course slope up against their generall fall. [7] Many thanks for Sir Charles Bunbury’s⁵ criticism.⁶ It is curious, that if I had trusted to the plants I observed myself on the mountain & had not quoted from Motley⁷ at all, I should not have mentioned any of the genera to which Sir C[harles] .B[unbury]. objects. I would remark however that the occurrence of Lobelia, Oxalis &c. in S. Africa & or Mexico, does not in the least affect the conclusions drawn from their presence on the Java mountain tops, because they must have entered Java, not across wide oceans, but across the plains & lowlands of India where I believe they do not occur. Impatiens in India is a mountain genus, seldom occurring at less than 4000 ft. elevation, & when the mean temp. is above 65° or 70°.

It is however decidedly wrong to call these European genera, & I will alter it [8] if possible in the next edition.

I have not yet had my final revises from the Quarterly so I will suppose the Review will be late again.⁸

Believe me | Yours very truly| Alfred R. Wallace [signature]

Sir Charles Lyell Bart.