262 W. A. E. Usshcr — Pleistocene Geology of Cornwall. 



peat, which had accumulated around them during the last stages of 

 their existence, it was some time before forestial growth in inland 

 districts succumbed to unfavourable climatal conditions, and still 

 longer before the succeeding undergrowth gave place to the bare and 

 shrubless character presented by so large a part of western and 

 central Cornwall now. 



Although it seems only reasonable to regard the deposition of 

 metallic detritus, as now going on, wherever the stream channels are 

 traversed by tin veins, this process is so insignificant that as a whole 

 the stanniferous gravels must be referred to a period considerably 

 posterior to the raised beach formation, and, either long after the 

 culmination of the elevation during which Head was accumulated, 

 or in part synchronous with its accumulation, when, through greater 

 elevation and increased rainfall, the force and volume of the streams 

 was greater. The commencement of the forest growth is also in- 

 definite, but subsequent to the accumulation of the Head, during the 

 prevalence of a subsidence which produced conditions unfavourable 

 to the existence of the tin floods as they became more suitable for 

 its extension. So that the forest growth may have begun before the 

 stream tin floods dwindled away, and the latter may have been 

 partly contemporaneous with the Head. Whilst marine sediments on 

 the forest bed or tin ground in estuarine sections (3, 5 A, G, E, 6 A, 

 B, C, 8 A) prove the last great movement to have been one of sub- 

 sidence, the more orderly arrangement of the deposits ; the general 

 absence of heavier far-borne detritus ; the entire desertion of parts of 

 their old channels by some of the present streams, indicate the 

 gradual prevalence of conditions more akin to those now prevailing 

 than to those in operation during the deposition of the stanniferous 

 gravels. 



The growth of trees, some very old, on the surface (5 C, -D), shows 

 that the latest of these changes must have been some time in opera- 

 tion, whilst the presence of human remains at great depths beneath 

 the surface, at Carnon and Pentuan, and the tradition respecting St. 

 Michael's Mount, would seem to justify the belief that the period in 

 which the forests were finally submerged, although geologically very 

 recent, is yet prehistoric. 



As the subsiding movement gradually enabled the sea to circum- 

 scribe the forest tracts on its old fore-shore, the beach materials 

 pushed forward would finally tend to bar the drainage of the valleys 

 opening on the coast, and to convert the low lands into peat mosses, 

 forming round the surviving trees till the further advance or disper- 

 sion of the beach dams permitted the sea to regain its old coast-line, 

 entombing the forest fringes and their peaty surroundings beneath 

 its sands. Eliminate from this all changes of level by internal 

 movements, and explain the entombment of the forests by the 

 lowering of level consequent on removal of gravel bars releasing 

 the pent-up drainage, and the low district theory is presented. 

 Without changes of level, however, it is perfectly untenable as 

 applied to Cornwall, where the stream tin gravels indicate a greater 

 elevation of the land (5 B), as at Carnon and Kestronguet Creek (6 A, 



