Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/254



The appearance of these specimens is not very inviting; but they are not without interest.

The pebbles of hard chalk are probably the remnants of the bed which immediately covers the green sand and the gault, of which these are either fragments rounded by attrition, or they are the nodules peculiar to the lower chalk, (as may be seen in Wiltshire,) washed out of the bed itself, which is disintegrated.

No. 2 is probably a variety of common flint; specimens of this substance are not uncommon, in which there is an appearance, as in a Scotch pebble, of alternate layers of deposition. The action of air and moisture might render these natural divisions more visible, just as slates are obtained by the exposure of the blocks of fissile stone to the weather.

As other beds, besides the London clay, contain septaria, we cannot say from what bed the fragments of this substance are derived; nor will any of the specimens previous to No. 6, furnish any data for guessing the nature and direction of the current, which has heaped together this mass of confusion.

The mass of greenstone nearest to Cambridge is found in the toadstone beds of Derbyshire, to some specimens of which the pebble No. 6, bears a close resemblance. Few of these pebbles weigh less than 8 ounces.

The large oyster is the same with fig. 7 and 8 of Pl. 8 of Townshend's work, and belongs to the bed which underlies the coral rag.

The large belemnite is peculiar, I believe, to the lower oolite.

The bassetings of the three last mentioned beds, that is, of the toadstone, the coral rag and the lower oolite, are found (at least in England) only in a direction west of Cambridge; so that we are led to explain the accumulation of these alluvia by the agency of a powerful current flowing from west to east.