W. A. E. Ussher — Post-Tertiary Geology of Cornwall. 103 



Agnes, must be regarded as the earliest traces of superficial deposits 

 as yet observed in Cornwall. 



Gravels of Crousa Down and Croioan. — On Crousa Downs, Lizard 

 District, a patch of rounded and subangular quartz gravel " occupies 

 an area of about half a square mile at a height of about 360 feet 

 above the sea" (Report on Geol. Corn, and Dev. p. 896). 



The Eev. E. Budge (Trans. B. G. S. Corn. vol. vi. pp. 1 and 91) 

 describes the deposits, generally, as extended layers of fine yellow 

 gravel, with a quantity of quartz pebbles, exposed in pits 10 to 12 

 feet deep in places, near the road leading to Coverack. The 

 character of the sections is given thus — Black peaty soil containing 

 small angular quartz stones about 6 inches thick, upon layers of fine 

 and very coarse gravel alternating in no very determinate order, 

 containing quartz pebbles of very irregular form, some as large as a 

 man's head, but for the most part not exceeding 2 to 3 inches in 

 length. The Crousa Down gravel rests on Diallage rocks. 



A similar occurrence was noticed by Mr. Tyack (62nd Ann. B. Geol. 

 Soc. Corn. p. 176, etc.) at Blue Pool in Crowan. The pebbles covered 

 an area of 800 yards from north to south, and 500 from E. to W. 

 They are scattered over the surface, are well worn, and vary in size 

 from large boulders to the dimensions of hazel nuts. The gravel is 

 400 feet above the sea, it rests on yellow clay. As at Crousa Down 

 the quartz is such as would be furnished by veins in the Killas, the 

 pebbles of schorl being very few, and the occasional granite frag- 

 ments angular ; yet the Killas districts near Crowan are at a much 

 lower elevation than the granite on which the pebbles are found. 



These quartz gravels appear to have been derived from quartz- 

 iferous Killas, either by direct transport of aqueous agencies suffi- 

 ciently protracted in their operation to allow of the comminution of 

 the slaty matter, or indirectly by the disintegration and redeposition 

 of a quartz conglomerate rock of Pakeozoic age. Beferring to the 

 derivation of the Crousa Down Gravel, the Bev. E. Budge illustrates 

 the prevalence of quartz veins in the Killas to the north by citing 

 the occurrence of masses of quartz in the slates near Nare Point, 

 whence they can be traced for some miles along the line of strike ; 

 and of a quartz vein 10 feet wide on the south-west of Carne, in St. 

 Anthony parish. 



Between the Loo Pool and Marazion, on the top of the cliffs, near 

 Trewavas Head, small flint and quartz pebbles occur in the soil, and 

 do not appear to extend more than a few paces inland. As their 

 height above the sea and the adjacent configuration preclude the 

 possibility of their being the relics of a raised beach, I am forced to 

 conclude that they are either traces of gravels somewhat similarly 

 situated to those of Crousa Down and Crowan, or that during 

 exceptionally severe gales some of the smaller pebbles of the beach 

 below had been from time to time carried upward in the spray and 

 landed on the top of the cliff. 



Deposits of St. Agnes. — (Report, etc., p. 258). De la Beche was 

 disposed to regard the sands and clays which nearly encircle the 

 higher parts of St. Agnes Beacon as " the remnant of some supra- 



