270 MR. T. CODEINGTON ON ST7BMEEGED EOCK- VALLEYS [Aug. 1898,. 



BkuUs. The tin-ground consisted of rounded masses of tin-ore, 

 sometimes unmixed with any other material, and rounded fragments- 

 of slate, granite, and quartz. In other sections in this valley the 

 tin-ground is descrihed as containing angular and suhangular blocks 

 of granite-rocks, slate, elvan, etc., with tin-ore in large rounded 

 masses, or in the state of gravel and sand. The upper surface of 

 the tin-ground was perfectly even, falling seaward with the valley. 



At a short distance below Higher Camon, about 3 miles from 

 the mouth of the creek, a timber viaduct of the Palmouth Eailway 

 afforded a section of the valley. The rock-bottom is a few feet below 

 the level of low water, and is now covered with 24 feet of gravel 

 and silt. Originally the tin-ground in this part of the valley was 

 immediately overlain by fluviatile deposits of recent age. 



The streams flowing into the Carnon rise in the Gwennap and 

 Stithian Hills, from 4 to 7 miles off, and about 600 or 700 feet abore 

 the sea, the furmer hills being of clay-slate and the latter of granite^ 

 The Palmouth estuary, opposite Eestronguet Creek, is 72 feet deep, 

 with a sand- or gravel-bottom. It deepens seaward to 114 feet at 

 the narrow entrance, outside which the bay is shallower, and the 

 depth is not so great as that of the rock-bottom in Eestronguet 

 Creek for a mile out. 



Close to Penryn the crossing of a creek furnished a section of a 

 regularly- shaped rock-valley, 24 feet below the level of low water, 

 filled up to that level with silt. 



III. Geneeal Obseevations* 



About as far north of Milford Haven as Plymouth and Falmouth 

 are to the south of it is the old rock-valley of the Mersey, filled 

 in with Boulder Clay and Glacial deposits, to which Mr. Mellard 

 Eeade ^ first called attention, and which has since been traversed by 

 the Mersey Tunnel and the Yyrnwy Aqueduct. 



On the western coast of "Wales the valleys opening into Caerdigan 

 Bay are striated by land-ice, and near Barmouth, where, as already 

 mentioned, the rock -bottom of the Mawddach Valley has been 

 traced to 40 feet below the level of low-water, striated rocks are 

 exposed at low tide. Sir A. Eamsay long ago pointed out that 

 there are well-marked glacial striations on the rocks of the coast of 

 Pembrokeshire, and that Boulder Clay with ice- scratched stones is 

 common there, and occurs here and there all over South Wales. 

 Prof. David has described in detail the evidences of glacialion in South 

 Brecknockshire and East Glamorgan.^ He has shown that Boulder 

 Clay containing rocks from the Brecknockshire mountains occurs 

 in the Taff Valley down as far as Treforest, and also in the Ely 

 VaUey as far south as St. Pagan's and St. George's, near Cardiff, 

 where it is about 20 feet above sea-level. He observed an extensive 

 striated surface of Coal Measure sandstone under the Boulder Clay 



1 Proc. Geol. Soc. Lirerp. 1873, p. 42. 



^ Quart. Journ. Geol. Soc. vol. xxxix (1883) p. 39. 



