Page:A Treatise on Geology, volume 1.djvu/251

 CHAP. VI.

But the most peculiar characters belong to the calcareous rocks, which are, of all the limestones known (excepting some in the tertiary deposits), the softest and most earthy. Not that the whole mass is correctly described by the term chalk, as technically applied by geologists; but yet a large proportion of the rock would be so termed even by ordinary observers, from the whiteness and comparative softness of it. In the lower parts, green grains are common; at the base in Lincolnshire and Yorkshire, a red band of from 6 to 12 feet in thickness is traced. Throughout the lower and indeed the greatest part of the chalk in Yorkshire, flint nodules occur in layers; but in the south of England they are nearly confined to the "upper chalk," in which they form layers 4 to 6 feet apart. At Sudbury, flint laminæ occur in the planes of stratification, as at Meudon near Paris.

Stratification.—The clearest possible evidence of regular deposition from water is found in all the rocks of this system, but in few instances are either beds or laminæ traceable so clearly or for such distances as among the older formations. In the green sands, beds are seldom clearly traceable, except where, as in the Isle of Wight and at Folkstone, argillaceous beds occur below and above, and are interpolated among the sands, or where, as at Maidstone, Hythe, and in Lincolnshire, bedded limestones necessarily introduce this structure among the sands. In other cases the layers of chert nodules, or thin chert beds, mark the successive stages of deposition: where none of these causes exist, oblique lamination, and concretionary geodes and other arrangements of oxide of iron, render it almost vain to look for stratification.

The golt clays are sometimes laminated (Speeton, Folkstone), and often, by the courses of small nodules, or by interposed beds of sand, show proofs of successive deposition.

The chalk is only partially bedded, and not at all laminated: its slow, and quiet, and intermitting