216 Mr. Webster on the Si rat a lying over the Chalk, 



tion ; 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 circum- 

 stances, 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 SQ 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 fre- 

 quently 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 



