My dear Lyell

I find I do not know enough of case to enter in your difficulty about subsidence.— Do you suppose the Jura had its present form, when the terrestrial quadrupeds were embedded?¹ if not, the subsidence need not have been so very great, & the Jura might have partly assumed its present form during that movement & its subsequent elevation when scratched by ice.— Do you make too sure of the period when ice floated being cold— pray remember in very latitude of Lake of Geneva, I forget how many miles of side of mountain is covered with ice, & the sea-channels are choked with icebergs—& the forests more resemble those of Brazil, than those of England.—²

I dont look at bridge of ice, (or the subsidence, or the absence of shells, for I think I out-Lyell Lyell) as any great difficulties.— but the whole turning point of the case appears to me to be, that he who knows most of glaciers, says most distinctly that the Jura erratics are totally distinct from those in valleys of Alps, which are moved in moraines.³ It strikes me as monstrous the expansion & contraction of a sea of ice carrying rocks from a small central point; surely the expansion would rather break up edges at foot of the peaks,—one can clearly see there is some limit,—to this action, if a point of rock a yard square projected out of enormous field of ice, no one would pretend that the ice would travel away on all sides from it.— Indeed I well remember in some of Agassiz’s earlier papers he says the ice expanded in the line of the Great Swiss Valley & hence the scratches had this direction⁴ but I see nothing about this in present work.—

I hope there are no perched rocks on Jura, as the more I think of that the stronger the argument appears for sheet of ice.— Do you know certainly whether Agassiz actually found the ‘creux’ or caldron under the existing glaciers in Alps— I can only remember, & could only find on casual look, that he found furrows— These caldrons appear to me far the most inexplicable part of case under every hypothesis—

I quite agree with you about putting the difficulties on both sides—

I dont know what reasons there are for supposing the Pentlands were dry land within any reasonably short time anterior to the elevation during ice time.—⁵ I presume you would not object to half a dozen oscillations since the secondary period—

Ever yours | C. Darwin