534 



VARIOUS LEVELS AT WHICH THE SHELLS ARE FOUND. 



The conchologist will not fail to remark, that the last list presents an intermixture of 

 several species common to our shores, with an Oliva and the Bulla Ampulla, shells now 

 only found in warm latitudes. However singular it may at first appear, this fact is not 

 inexplicable even by reference to existing causes, for if the view which we have been 

 advocating concerning the former relations of sea and land in this tract be sustained, 

 it follows that the climate of the Vale of Worcester, when a strait of the sea, probably 

 approached to a comparatively warm character. Such shells, therefore, as the Oliva 

 and Bulla might then occasionally, though rarely, occur. This view will be made more 

 intelligible in the sequel. 



The drift described in this chapter is found at very various levels. In the plains of 

 Cheshire it lies from twenty to a hundred feet above the sea. In the hills-around Shrews- 

 bury and in Staffordshire it occurs at heights varying from 150 to 500 and 600 feet, while 

 on the edge of North Wales it is nearly 1 700 feet \ Before, however, we attempt to con- 

 sider the method by which detritus formed of the same materials, and containing the same 

 species of shells, and therefore evidently distributed at the same period, could have been 

 placed at such very different levels, we must endeavour to work out an essential part of 

 the problem, the position of the great erratic blocks. The shells and small materials may 

 evidently have been accumulated in an estuary of the sea under ordinary circumstances, 

 but were the accompanying great erratic blocks deposited also by tidal action ? To this 

 consideration the following chapter is principally devoted. 



and on a recent occasion Mr. Strickland accompanying Mr. Allies found with him the Anomia Ephippium, having 

 much of the nacre preserved, together with Purpura Lapillus, Turritella Terebra, &c. 



The other shells were collected by a very honest and veracious person (the head workman). It must be re- 

 collected that the superior mass of coarse gravel is seldom cleared away to the low level of the running sand 

 in which the shells occur, and hence they can be detected at rare intervals only. Finding the shells well pre- 

 served at Kempsey, where they lie in wet sand near the surface of the red marl; and on the other hand much 

 decomposed and very fragile at Powick, where they are in dry gravel, Mr. Allies has suggested an explanation 

 which seems to me well worthy of consideration. Where the gravel is porous and the water percolates freely, 

 the shelly matter, he submits, being subjected to alternations of moisture and drought, is soon decomposed ; 

 but where it is imbedded in sand, which is constantly wet and not exposed to atmospheric variations, the most 

 delicate species are well preserved, even retaining some portion of their colours and the "nacre" of the shell. 



The same explanation applies, indeed, under modifications to all the shelly accumulations with which I am 

 acquainted in Lancashire, Cheshire, Shropshire, Staffordshire, &c; for wherever the accumulation is argillaceous 

 and impervious to air and water, as in the new rail-road excavations in Cheshire, the hinges, &c. of the most 

 fragile shells are well preserved, while in the porous loose gravel of Preston and other places we find little else 

 besides the Turritella Terebra and those shells which best resist destruction. 



1 In a very instructive letter, Mr. Trimmer informs me, that in addition to the case of Moel Tryfan, he has 

 detected sea shells mixed with granite pebbles, both in low cliffs on the south-western coast of Caernarvonshire, 

 and also on Moel Fabau near Bangor, eight miles distant from the coast. 



