Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/226

216 ; most of them appearing to have undergone but little change, and some are even scarcely to be distinguished from recent shells.

The situation of this bed, distinctly, placed above the vestiges of a freshwater lake, would seem to indicate some great revolution in the relative level of the land and sea, since the time of the marine deposit which we have already considered; and the above circumstances, combined with its position as regards the vertical beds of Alum bay, point out in strong characters a later period.

The spot where this stratum is best examined is Headen, near Alum bay. It there appears half way up the cliff; is about 36 feet thick, and dips a few degrees to the north. It passes from thence all round Totland and Colwell bays.

The substance of the stratum is chiefly marl of a light greenish colour, and the fossil shells are so numerous that they may frequently be gathered by handfulls, and are in general extremely perfect. I did not observe that the several species occupied separate beds, although they were much thicker together in some places than in others, and were then oftener accompanied by rounded nodules of greenish indurated marl. From the delicacy of the shells and their perfect preservation, it is evident that they could not have been brought from great distances, but must have lived near to the spots where they are now found. This greenish marl is separated from the upper freshwater formation only by a bed of sand a few inches in thickness.

In Colwell bay, at a fissure called Bramble's chine, there is in this stratum a very large bank of fossil oyster shells; the greater part of which are locked into each other in the way in which they usually live, and many have their valves united. It is therefore clear that this oyster bed has never suffered a removal. This stratum may be traced, with various interruptions, all round