W. A. E. Ussher — Post- Tertiary Geology of Cornwall 107 



in their present form is probably largely due to this protecting 

 envelope of talus shed from the adjacent Beacon hill, which ex- 

 ceeds 600 feet in height. 



From such local materials as the granite on the west of the 

 Beacon, the Elvan Course to the north of it, and the Killas, the 

 sands and clays seem to have been formed. 



The position of the deposits with reference to the present coasts, 

 and to the high land of the Beacon, and the cliff-like sections and 

 waterworn hollows noticed in some parts, would seem, "as De la 

 Beche suggests," to justify a marine origin, but with them "the 

 resemblance to the raised beaches appears to terminate" (Report, 

 page 258.) The interchangeable characters of the sands and clays 

 are more in accordance with the irregular deposition of a stream, 

 subject to fluctuations attendant on meteorological changes, than with 

 the more uniform sorting action of a coast-fringing sea. The very 

 local character of the basement gravels is also against the admission 

 of a marine origin. As an entirely new system of drainage has 

 been moulded since, the deposits were thrown down, proximity to 

 the present coast-line is no argument in favour of marine origin or 

 former proximity to the sea. 



Fluviatile agencies, which have produced similar effects in wear- 

 ing the surface of the shelf in stream-tin sections, coupled with the 

 weathering and water- wear of a vertical face, slickenside, or joint, 

 might, in the absence of further evidence, explain the phenomena 

 of the smoothed surfaces, water-worn hollows, and old cliff face 

 mentioned by Messrs. Kitto and Davies. Had such action pre- 

 vailed for a long period in an old line of drainage down which the 

 coarser detritus had been swept, the damming up of the old 

 stream course and selection of a new one above the present site of 

 the deposits, would tend to the formation of a lake in whose quiet 

 waters the finer debris of the adjacent land borne down by rills and 

 streamlets would have been filtered, and have settled down in the 

 form of sand and clay. 



The isolated positions of the deposits of Crousa Down, Crowan, 

 and St. Agnes, afford no clue as to their relative ages. Yet this 

 isolation justifies me in classifying them together as the oldest super- 

 ficial deposits as yet noticed in Cornwall. An entire bouleversement 

 of the levels of their respective districts has taken place since their 

 formation, and all traces of synchronous deposition have been swept 

 away in the elaboration of the present drainage system. As they 

 can only be regarded as relics of much more widespread deposits, 

 the possibility presents itself that we may have in them the traces, 

 in situ, or re-distributed, of Tertiary or even late Cretaceous deposits, 

 presenting a different aspect to that in other areas through de- 

 pendence on local sources of supply. During the vast period that 

 intervened between the Culm-measure rocks and the Pleistocene 

 Age, it is unreasonable to argue from the absence of deposits of 

 intermediate age that Cornwall was never invaded by Secondary or 

 Tertiary seas. 



On the Occurrence of Flints in Cornwall. — De la Beche (Report, 



