356 ME. H. B. MILXEE OJf THE [vol. IxXVlii, 



(c) St. Keverne. 



In the Croiisa- Common gravels there is a much wider variation 

 noticeable, and the deposits are coarser and more heterogeneous in 

 composition than in the preceding cases. The material consists 

 largely of subangular fragments of quartz, occurring both as 

 grains 025 mm. in diameter and as pebbles 2 or 3 inches in 

 diameter, with every intermediate grade present ; associated with 

 the quartz are pebbles of slate of the ' killas ' type, schorlaceous 

 fragments, and pebbles of rocks recognizable in the Lizard series. 

 Impersistent seams of finer sand and occasionalh^ §'i*^y clay accom- 

 pan}" the gravels, but the}' are by no means common. Reference 

 must be made to the contortions and local folds exhibited hj these 

 gravels : this feature is certainly most sti-iking in the pit adjacent 

 to the main St. Keverne road (p. 352), but it would seem to be more 

 easily explicable as due to current-bedding of coarse detritus in 

 shallow water, than as the result of later subsoil movements.^ The 

 coarse texture and the very mixed character of these deposits tend 

 to accentuate such inequahties of bedding ; but, when examined 

 closely, their true neritic character is quite apparent. The amount 

 of material that has to be sifted in order to produce a sand 

 concentrate for petrographical investigation is more than double 

 that used in the other cases ; this in itself bears testimony to the 

 nature of these o-ravels. 



{d) Polcrebo. 



Mr. J. B. Hill has already pointed out a similarity existing 

 between the Polcrebo gravels and those of Crousa Common, in so 

 far as the quartz-pebbles are concerned, a feature remarked by 

 W. Tyack in his original description of the former deposit.- The 

 greater mass of the Polcrebo material is quite unlike that of 

 Crousa Common, however, and consists of Avell-worn rounded 

 pebbles of quartz and granite, the latter of local origin. These 

 pebbles are extremely variable in size ; Tyack says that ' they are 

 of all sizes, from boulders as large as a pumpkin to pebbles as small 

 as hazel-nuts.' Sand in association with the gravels is of rare 

 occurrence, and what fine-grained material (2 mm. in diameter) 

 there is. has been derived solel}^ from the weathering and disinte- 

 gration of the pebbles themselves. But for their elevation at 

 iSO feet above sea-level, there is little lithologically to suggest a 

 relationship to the deposits under consideration, although their 

 mode of occurrence is certainly sisrnificant. 



^ ' The Geology of the Lizard & Meneage ' Mem. Geol. Siirv. 1912, p. 230. 

 - Trans. Eoy. Geol. Soc. Cornwall, vol. ix (1875) p. 175. 



