T. BELT ON THE DRIFT OF DEVON AND CORNWALL. 83 



formation. On the hills large stones are found scattered over the 

 surface ; in the Lowland deposits they occur at the bottom of the 

 gravels. In Cornwall, the stream-tin is found at the bottom of the 

 gravel, along with rocks of quartz of great size. In 1830, Mr. 

 Joseph Carne urged that the occurrence of the stream-tin invariably 

 at the bottom of the gravels proves that the whole of the materials 

 had been in motion at once, and that these deposits furnish the 

 strongest evidence of a sweeping inundation *. 



In this view Mr. Carne followed Prof. Sedgwick, who, fifty years 

 ago, had satisfied himself that the gravels had been spread out by a 

 great debacle f. Mr. Carne supported this theory with great ability, 

 and referred to the fact that all the productive stream-tin works 

 were situated in valleys opening to the south, whilst the richest tin 

 veins are situated near the northern coast, and concluded that a deluge 

 of water from the north had swept the tin into the southern valleys. 

 Sir H. De la Beche fully concurred in this conclusion, and furnished 

 additional arguments in its support. He says that any detritus that 

 existed previously to the deposition of the stanniferous gravels ap- 

 pears to have been "fairly washed out by a mass of water rushing 

 over the land, rolling and driving various loose materials before it, 

 and allowing the tinstone, from its greater specific gravity, to be 

 strewed along the bottom " J. 



In Devonshire, in every valley there is a repetition of the same 

 facts — a scouring out of their upper reaches of all loose materials, 

 which are spread out in great sheets towards their mouths. In the 

 valleys of the Teign and its tributaries, there are some of the largest 

 accumulations of gravel in Devonshire ; and we have distinct evi- 

 dence that it was not deposited during the excavation of the 

 valleys, but that the latter were formed long before Miocene or prse- 

 Miocene times ; for at, and near, the junction of the Bovey river 

 with the Teign are found the Bovey-Tracey lignites of Miocene age, 

 lying in the valleys and covered by the lowland gravels. The Bovey- 

 Tracey lignites have been bored through to a depth of 200 feet with- 

 out the bottom of the old prae-Mioeene valley being reached. The 

 drift-beds extend up the sides of the valleys, and to the tops of the 

 ranges bounding them. 



3. Theories of origin. — The facts to be explained, not only in 

 Devon and Cornwall, but, as we shall see, over the whole of the 

 south of England, are : — (1) gravels containing travelled boulders, 

 on the sides and tops of hills up to about 1200 feet above the sea ; (2) 

 denudation of the surface-gravels from an intervening tract between 

 the Upland and Lowland deposits ; (3) in the lowlands, especially 

 in the valleys within 100 feet above the level of the sea, a wide 

 spreading-out of gravels that show signs of sudden and tumultuous 

 deposition. 



Subaerial denudation could not produce the hill-gravels containing 

 rocks brought from a distance. Nor could the deposits lying on the 



* Trans. Eoy. Soc. Cornwall, vol. iv. p. 47. 



t Annals of Philosophy, vol. ix. p. 241. 



| Geology of Cornwall, pp. 396, 398, & 410. 



o 2 



