CORNISH POST-TERTIARY GEOLOGY. 39 



in some places, and of large masses of slate in tlie gravels (from 

 the disintegration and fall of rugosities in the river-bed or banks) 

 in others, renders it possible that the water-worn face and out- 

 standing pinnacle of the cliff may be due to fluviatile agencies. 



The preservation of the sands and clays is probably due to an 

 envelope of talus shed from time to time through the weathering of 

 the high land of the Beacon (620 feet above the sea), and repre- 

 sented by the overburden or Head in the sections. This Head was, 

 in all probability, accumulated after a considerable denudation of 

 the deposits had taken place, so that it rested directly on the sands 

 on tlie edges of the patch. Such appearances as the inosculation of 

 the Head with the upper bed of sand in pit No. 3 might be explained 

 by rain floods carrying debris over an eroded surface of sand, and by 

 the penetration of winter frosts, causing the intrusion of the earthy 

 material in the hollows of the surface on which it rested. I see no 

 reason to doubt the contemporaneity of the overburden with the 

 Head on the cliff-line. 



The local character of the pebbles in the deposit might be con- 

 sidered as an objection to the fluviatile theory. I assume, however, 

 the former existence of a stream draining districts of similar con- 

 stitution to the present surroundings of the patch, and account for 

 the variety of the sediments not only by fluctuations in the condition 

 of the stream, but by its conversion into a tarn through the stopping 

 of its seaward outlet. The varied alternations of sand and clay 

 would then result from the deposition of a stream, whose finer 

 sediments would be precipitated in the deeper parts of the lake, 

 their precipitation being interrupted by influxes of coarser material 

 when the stream was swollen or the dam temporarily removed. 



Their isolation prevents one from assuming that the formation of 

 the sands and clays of St. Agnes was contemporary with the 

 deposition of the Crousa Down and Crowan gravels, but in the 

 absence of connecting links, they may be classed together within a 

 period ranging from Tertiary to early Pleistocene times, when the 

 contour presented a vastly different aspect from its present outlines, 

 before the selection of the present lines of drainage. 



