To W. E. Darwin   [1 March 1872]¹

My dear W^m

It was a capital thought of yours, to get the chalk analysed.² Immediately on getting your note I called on David Forbes, who is the one man in England who has carefully analysed many specimens of chalk.³ He says that from 1 to 2 per cent of earthy matter is ⁠⟨⁠a⁠⟩⁠ fair average for the upper chalk with flints. In the lowest & most earthy beds of chalk without flints there is as much as 4 or 5 per cent. Therefore he concludes that the specimens analysed by Capt Parsons must certainly have become penetrated with surface mud.⁴ This seems to remove the wonderful difficulty of the layer of mould not being thicker at the bottom of your valley. I suppose the chalk about Winchester must be very pure, or unusually porous; for about Down the residue from the dissolved chalk evidently accumulates on the surface, & does not to any great extent percolate into the solid mass—

yours affecty | Ch Darwin