352 ME. H. B. MILIfEK OX THE [vol. Ixxviii, 



{c) St. Keveriie. 



Resting on the Lizard ' platform ' at a height of 364 feet above 

 O.D., and situated a mile and a half due south-west of the village 

 of St. Keverne, are the Crousa-Common gravels hrst noticed by Sir 

 Henry De la Beebe in his report of 1839. Unlike the similar deposits 

 of St. Agnes and St. Erth, the extent of this outcrop is easy of 

 definition. It consists of a rectangular mass 1 mile long b}^ half a 

 mile broad, lying upon the eroded surface of the Lizard gabbro, 

 the conspicuous feature of this area. As Mr. J. B. Hili has pointed 

 out,i the extent of the deposits is naturally defined by the sharp 

 change in vegetation noticeable as the boundaries between the 

 sands and the igneous rocks are traversed ; in the former case the 

 marked development of gorse, heather, and pine contrasts strongly 

 with the more fertile grassy soil yielded by the gabbro, and the 

 typical weathered blocks of this rock scattered everywhere over 

 the surface of its outcrop constitute a further element of differen- 

 tiation between the two. 



Both natural and artificial sections in these sands are common, 

 but only one pit is at present worked, the others being overgrown 

 and often under water. The best exposure is that in a shallow 

 quarry adjoining the main road to St. Keverne, immediately east 

 of the 9th milestone from Helston. This pit shows a section of 

 coarse sands and gravels varying from 8 to 10 feet in thickness, 

 and capped by a few inches of mixed surface-detritus and peaty 

 soil. The deposits are remarkable for their varying texture, the 

 strong current-bedding exhibited, and for the quantity of big 

 pebbles and even boulders associated with them. Their base is not 

 seen in this particular pit, although it cannot be far below the 

 floor of the present Avorking (1920), as in wet weather the pit is 

 badly flooded, pointing to the existence of impervious clayey rock 

 at no great depth, such as would be yielded by the superficial 

 decomposition of the gabbro. In some of the older abandoned 

 pits on the north side of the road in the wood, blocks of gabbro 

 maj^ be seen in dry weather protruding from underneath the sandy 

 floor, thus showing the vertical extent of the deposits. 



Xo fossils have ever been found in the Crousa-Common deposits, 

 and their Pliocene age has been inferred from their relations to 

 the Lizard 'platform,' a remnant of the characteristic Pliocene 

 features of the West Country. 



{d) Polcrebo. 



The curious pebble-bed found at Polcrebo near Crowan, -1 miles 

 south of Camborne, originally described by W. T^^ack - and later 

 referred to by Mr. J. B. Hill,^ is of very limited extent ; it consists 

 principally of well-rounded [pebbles of quartz, resting at a height 



1 ' The Geology of the Lizard & Meneage ' Mem. GeoL Sury. 1912, p. 230. 



^ Trans. Eoy. Geol. Soc. Cornwall, vol. ix (1875) p. 177. 



3 ' The Geology of Falmouth & Camborne ' Mem. Geol. Siu-v. 1906,, pp. 88-89. 



