On the “Chalk-Rock,” the Topmost Bed of the Lower Chalk, in Berkshire, Oxfordshire, Buckinghamshire , &c.

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The bed to which I have elsewhere given the name of “Chalk-rock,” I believe to form the division between the Upper and Lower Chalk, and to be the topmost bed of the latter. I have described it as “hard blocky chalk, jointed perpendicularly to the plane of bedding, with lines of irregularly shaped, hard, calcareo-phosphatic nodules, which are green outside, but cream-coloured within.” It breaks with an even fracture, rings when struck with the hammer, and is of a pale cream-colour (the nodules being darker than the rest). For some time I thought that this bed had escaped the notice of geologists; but Mr. Prestwich tells me that he has long known of it. It has also been noticed by Mr. Evans, of Hemel Hempstead, and Mr. W. Cunnington, of Devizes. However, I believe that no account of it has yet been published, with the exception of the short description above referred to. My own observations have been confined to the counties of Wilts, Berks, Oxon, Bucks, and Herts—that is, to the northern side of the western part of the London Basin,—in which area I found that the Chalk-rock reaches its greatest thickness to the west, gradually thinning eastwards.