- Clay with limestone and bituminous shale, containing the Kimeridge coal.
The strata of Alum bay, now seen in a vertical position, must have been originally quite horizontal, or nearly so. For it is not only extremely improbable that beds of such materials should have been formed[errata 1] in any other manner, but a circumstance which I noticed in Alum bay renders certain the original position of the sand and clay strata. In one of the vertical beds, consisting of loose sand, are several layers of flints, extending from the bottom to the top of the cliff; these flints have been rounded by attrition; are from one inch to eight inches in diameter, and appear to have belonged to the chalk. Now it is inconceivable that these flint pebbles should have been originally deposited in their present position; and they distinctly point out the original horizontality of this series. There are no signs of partial disturbance in the several beds, and it appears therefore that the whole have[errata 2] been moved together, either by elevation or subsidence into the vertical situation.
The strata of chalk have evidently suffered a change of position at the same time with the clay and sand; and since the vertical beds of Alum bay, G, are next to the stratum of flinty chalk, which, according to the regular order of superposition, is known to have been the uppermost of the chalk series, it follows, that they are of posterior formation. Moreover, the most northern of these vertical beds is a blue clay, agreeing with the London clay, a bed which always lies over the chalk.
- Other divisions of these strata have been made, and the cause of the differences has already been alluded to. Not every member of the series is perfectly continuous or of uniform thickness; so that when any one is wanting, beds, usually separate, are brought into immediate contact. In making a classification of these beds, those should be considered as principal members, which are most rarely deficient in an extensive tract of country.