274 MK. T. CODKUfGTON ON SUBMERGED EOCK-VALLEYS [Aug. 1898, 



to the tin-ground in Cornwall are no older than the submerged 

 forests. 



Por the deposits below there remains all the time back to the 

 period of the excavation of the valleys, and there is little to show 

 to what part of that time the several deposits belong. The gravel 

 and large stones over the Boulder Clay in the J^eath section, some 

 of the gravel with erratic boulders in the Wye cylinders, and the 

 coarse gravel with glaciated blocks in the cutting east of the Severn 

 Tunnel may, I think, be referred to the latter part of the Glacial 

 Period. The same may be said perhaps of the clayey gravel and 

 stones overlying the rock-bottom of the Severn at the bridge, though 

 if so the gravel overlying the peat in one pair of cylinders must be 

 newer than the rest. 



The clay and stones, clay and gravel, large stones and boulders 

 found overlying the rock-bottom in the Dart, at Millbay and at 

 Saltash, are probably of Glacial age, but they have been too little 

 seen for anything to^be said with confidence. Further information 

 may show, as at Coombe Lake and in the Tavy section, that they 

 are part of a continuous layer of * boulder-clay ' over the rock. 



The stream-tin gravels of Pentuan and Carnon are considered by 

 Mr. TJssher ^ to be newer than the raised beaches, and that view appears 

 to have been adopted by Sir J. Prestwich, who included these gravels 

 among * Head ' or ' Eubble-drift ' deposits.^ The reasons given by 

 Mr. Ussher do not seem to me to be conclusive. Stream-tin detritus, 

 or a gravel of local rocks and tin-ore of all sizes, occurs at St. Agnes 

 Beacon, near the north coast, 8 miles from Carnon, under alternating 

 sands and clays of uncertain age, but undoubtedly far older 

 than the raised beaches. They are 300 to 400 feet above the sea, 

 they appear to be unconnected with the present configuration of the 

 surface, and they are overlain by ' head ' or- rubble drift. De la 

 Beche considered that they belonged to a class of deposits that may 

 once have covered a considerable portion of the district, and wbich 

 have been since removed, with a large mass of pre-existing rocks, to 

 form the surface as we now see it. They are classed by him as 

 Tertiary, and by Mr. Ussher as ranging from Tertiary to early 

 Pleistocene times.^ Considering the small proportion that the tin- 

 stone bears to the containing rock, the degradation of the latter 

 necessary to produce stream-tin detritus must have been very great,, 

 and it is not surprising that it began so far back ; but the existence 

 of these old stream-tin gravels appears to nullify the conclusion that, 

 because stream-tin detritus is not found in the raised beaches, it is^^ 

 therefore more modem than they. The older gravels must have 

 furnished part of the materials for the stream-tin gravels of the 

 existing valleys, and that would account for the mixture of rounded 



1 ' The Post-Tertiary Geology of Cornwall,' 1879, p. 45. 



2 Quart. Journ. Geol. Soc. vol. xlviii (1892) p. 342. 



^ See also J. Hawkins, Trans. Eoy. Genl.^oc. Cornwall, vol. iv (1832) p. 137 y 

 De la Beche, 'Report on the Geology of Cornwall, Devon, & West Somerset,' 

 1839, p. 259; and Ussher, 'The Post-Tertiary Geology of Cornwall,' 1879, 

 pp. 12-39. 



