J. p. Johnson — Fossil and Recent Shells, Cornwall. 25 



-a struggle to hold the field between the accumulation of sea sand 

 driven up by the wind and the accumulation of rock debris and 

 loam under atmospheric erosion. In the cliff sections to the west- 

 ward of the Headland Hotel the Pleistocene blown sand and the 

 Head are rudely interstratified together at their junction, whiles at 

 a short distance away the Head is seen to pass almost insensibly 

 upward through loamy sand into the Holoceue blown sand above. 

 The former case is evidently due to the fluctuation in the dominance 

 of the one mode of accumulation or the other, during a gradual 

 change in conditions, as already suggested. There can be little 

 doubt but that this was caused by a gradual elevation of the land, 

 and consequent removal from the source of supply of the sand, 

 together with increase in erosion of the surface ; and when, after 

 the Head, the Holocene blown sands became dominant, to an almost 

 equivalent depression. 



One naturally speaks of these changes in the relative level of land 

 and sea in terms which refer to the elevation and depression of the 

 land. Kecent researches have shown, however, that the surface of 

 the sea is by no means the uniform level that we have been wont to 

 imagine ; and that these changes may equally well be due to local 

 fluctuations in its level. But for purposes such as that of the 

 present paper, one can be content to leave the question unsolved. 



V. — Notes on Fossil anb Eecent Shells obtained on a visit 



TO Cornwall. 

 By J. P. Johnson. 



THE following notes were made during the little time I was able 

 to devote to leisure while staying at Camborne in order to 

 study the methods there employed of mining and dressing the 

 tin-bearing rock. They therefore deal only with places within easy 

 reach of that town. 



The whole of this district is made up of extremely ancient 

 rocks, of which I must content myself with saying that they afford 

 a boundless field for those interested in the variation and alteration 

 of granite, in the killas through which it rises, and in the granite- 

 ,porphyry and other dykes with which these are traversed ; or for 

 those seeking to unravel the many problems connected with the 

 mode of occurrence of the tin and copper lodes. Hei'e and there, 

 resting on these old rocks, are scattered patches of comparatively 

 recent deposits, none of which date back to before the Neogene era. 



The most interesting of these is certainly the fossiliferous Pliocene 

 clay at St. Erth. All information concerning it was summarized 

 by Clement Eeid ^ in 1890, and increased in 1898 by Alfred Bell,^ 

 who also figured some of the more noteworthy shells. The deposit 

 is of very limited extent, and is situated in the valley, then probably 

 a strait, which connects Mounts Bay and St. Ives Bay. The 

 exposures, which I was enabled to examine through the courtesy 



1 " Pliocene Deposits of Britain" : Mem. Geol. Surv., London, 1890. 



2 " On the PHocene Shell-beds at St. Erth" : Trans. Eoy. Geol. Soc. Cornwall, 

 1898, vol. xii. 



