316 



PROF. J. PEESTWICH ON THE EAISED 



because all those he had seen from the lower valley-levels were 

 more rolled and worn. This bed, however, is not a high-level 

 Yalley Drift. We are here out of the Severn Yailey proper, and 

 there is no stream with which this deposit can be connected. Nor 

 do I take the red clay to be Boulder Clay, but merely a clay derived, 

 like the angular rock-fragments, from the older rocks and red marls 

 higher up the hill, whence also, if not from a glacial sprinkling, the 

 quartzite and old rock-pebbles may have been derived. This deposit 

 and others described by Mr. Symonds have in fact all the characters 

 of the angular drift and Head, which I have been describing, and 

 should be associated with them. 



To return to the South Coast. 



(13) Cornwall. — Beacon Hill, near St. Agnes, on the north coast 

 of Cornwall, rises to the height of 628 feet. Its slopes are covered by 

 a Eubble-drift varying from 6 to 20 feet in thickness, which is here 

 rendered more conspicuous than in other parts of Cornwall, from its 

 extending, beyond the limits of the slate-rocks, over the eroded sur- 

 face of the Tertiary sands and clays formiug the lower slopes of the 

 hill. The rubble consists of slate and granite-fragments with a few 

 quartz-pebbles, all of local origin, and is analogous in its character 

 to the debris which overlies the Eaised Beaches. Mr. Ussher sees 

 no reason to doubt the contemporaneity of this drift, which he terms 

 ' overburden,' with the Head on the cliff-line.^ 



As before observed, the Rubble-drift always proceeds from 

 higher ground above the Raised Beaches, whence it descends to the 

 sea-level, though it is generally cut off from the shore by subse- 

 quent encroachment of the sea. In a similar manner, when that 

 drift has been shed down the hill-slopes inland, it has descended 

 to the adjacent old valley-level ; and where the valleys pass 

 through lode-bearing rocks, the detritus carries with it its con- 

 tingent of those lodes, and it is this, which corresponds therefore 

 with the Head of the Beaches, that in Cornwall constitutes the 

 Stream-tin Gravel. The valleys themselves are of older date, and 

 were at that time of abnormal depth, for, in addition to the depth to 

 the base of the Stream-tin Eubble-drift, we must add the height 

 of the Eaised Beaches ; nevertheless, as in none of them has there 

 been found an ordinary worn and rolled river-shingle or sand, we 

 must presume either that the force of the stream kept the channels 

 clear (which in the case of such small rivers was hardly likely), or 

 else that the tin-gravel was propelled by such force as to sweep 

 and drive out any previous deposits. This was the opinion of He la 

 Beche,^ and is, I think, the most probable solution of the difficulty. 



The relation of the Eaised Beaches and Eubble-drift to the 

 Stanniferous Gravel is shown in the diagram, fig. 16. 



The Stream-tin detritus, which consists of angular and subangular 

 fragments, and often contains, like the Head, blocks of large size 



1 'The Post-Tertiary Geology of Cornwair (1879), pp. 13, 39. 

 ^ ' Eeport on the Geology of Cornwall, &c.' p. 398. Mr. Carne was the first 

 to direct attention to this origin of the Stream-tin Grayel. 



