WCP2162

[1]

Edinburgh

25 Nov[embe]r. 1849.

My Dear Mr. Wallace,

Many thanks for your letter. I shall send my letter to Nature in the course of a few days. I would have done so sooner but the head has been troubling me a good deal of late and besides I was half expecting that some Meteorologist[?] would remove his difficulty as it is more apparent than mine real. The conclusion Mr. Fisher draws from Pouillet [2] is perfect nonsense and Mr. Hoill's deductions from Dulong and Petit I do not believe to be correct.

In reference to the evidence of [1 word crossed out, illeg.] cold periods prior to the Miocene Age I think we have pretty good physical evidence of the existence of glaciation back to at least the Cambrian if not to the Laurentian Period. A great many of the facts I have detailed as you will remember in Chap. XVIII of Climate and Time and in Chap. XVII I have given [3] what appears to me to be good evidence why we have not more physical indications of former glacial periods.

Regarding the Palaentological evidence I know but little. I suspect that few Palaeontologists ever dreamed or expected to find Glacial animals in Palaeozoic limes. Perhaps if they had been on the outlook they might have found such evidence. Physical Geologists never found physical evidence until they act began actually to look for it. And I have little doubt that when Palaeontologists begin to reach for [4] evidence that they will find it.

But in regard to far back periods could we tell with certainty the class of animals that would indicate glacial cold? Of course I do not know but this you no doubt can tell whether such should be the case or not. We have in the Palaeozoic Limes well marked zones where life is scarce if not absent. This of course is not positive proof of cold but it is a strong presumption in favour of it.

Prof. Geikie says that in the 2^nd Edit[ion]. of Dana's Manual of Geology there is something [5] about Palaeontological evidence of cold periods during Pliocene ^the Palaeozoic Age.

I shall ask Dr. James Geikie if he knows of any.

By the way there is a very interesting article by Peach and Horne two of our men in the present number of the Luart.[?] jour[nal]. Geol. Soc. on the glaciation of the Shetlands. It is one of the most remarkable memoirs which have yet been written on glacial ice. It shows beyond all doubt that the North Sea was filled [6] with land ice from Scandinavia. Lately they have visited the Orkneys and gave them a thorough survey for drift and have found that these islands have likewise all been over-ridden by North Sea ice. And lately ^{just now} one of our men engaged on the Survey to the North of Aberdeen has found evidence that the North Sea ice passed up on the land at that place [7] bringing with it broken and glacialed shells from the bed of the sea.

I am | Yours truly | James Croll[signature]