Vol. 54'] I^ SOUTH WALES, DEVON, AND COENWALL. 269 



Cornwall. 



About 20 miles west of the Plymouth group of valleys is the 

 Fowey estuary, suggesting similar submerged valleys, of which, 

 however, there is no direct evidence. 



About 7 miles farther west the construction of the Gover Viaduct, 

 near ^t. Austell, furnished a section of the valley in which the 

 Pentuan stream-tin works were situated. The river rises near 

 Hensborough Beacon (1027 feet), and has a course of about 8 miles. 

 At Gover the rock-bottom of the valley is 179 feet above the sea, 

 and the present bottom is 39 feet higher. The Pentuan stream-tin 

 works extended from about 3 miles lower down the valley to within 

 ^ mile of the sea, a distance of more than a mile. At the upper end 

 of the workings the rock-bottom is above the sea-level, but near the 

 sea, according to the section given by W. J. Henwood,^ it is 36 feet 

 below the level of low water. The tin-ground there rested imme- 

 diately upon a clay-slate bottom, and consisted mostly of rounded 

 fragments of granitic rocks from tlie hills above St. Austell, with 

 angular fragments of greenstone and clay-slate. J. W. Colenso " 

 states that the tin-ground was distinguished by the streamers as 

 ' loose ground,' consisting of sand, pebbles, and stones, and ' tough 

 ground,' in which the whole was cemented together by yellow clay, 

 so as to render it difficult to separate and wash. The same writer 

 notices that the rock beneath appeared to be worn by friction, but 

 not that under the ' tough ground.' The surface of the rock was 

 irregular, while the top of the tin-ground was even, so that the 

 latter ranged in thickness from 3 to 6 or even 10 feet. Pooted in 

 the tin-ground were stumps of trees, to which oysters were attached, 

 at 30 feet below the level of low water, or 48 feet below high -water 

 spring-tides. A bed of silt, with leaves, roots, etc., succeeded, and 

 silt and sea-sand alternated for a thickness of about 52 feet. A 

 mile higher up the valley there was no sea-sand. 



Westward again is Falmouth estuary, with its numerous creeks, 

 one of which, Restronguet Creek, is the continuation of the Carnon 

 Valley, both of which have been extensively explored in the search 

 for stream-tin. In the middle of the creek, about a mile from its 

 mouth, near Point, a shaft was sunk through iron cylinders to the 

 rock-bottom of clay-slate at 64 feet below the level of low water.^ 

 The tin-ground, according to the irregularities in the surface of the 

 rock, was from 6 inches to 6 feet thick, and over it lay 63 feet of 

 silt and mud, with oysters and cockles. About | mile higher up the 

 creek, at Narabo Inlet, is the section described by W. J. Henwood,* 

 where the rock-bottom was 56 feet below the level of low water, 

 and the tin-ground was immediately overlain by a bed of wood, 

 moss, leaves, nuts, etc., 44 feet below the level of low water, con- 

 taining a few oyster-shells and animal remains, including human 



1 Journ. R. Inst. Cornwall, vol. iv (1873) pp. 243, 254. 



2 Trans. H. Geol. Soc. Cornw. vol. iv (1832) p. 30. 



3 Journ. R. Inst. Cornw. vol. iv (1873) p. 204. 



^ Trans. R. Geol. Soc. Cornw. vol. iv (1832) p. 58. 



