ASSOCIATED WITH THE PLYMOUTH LIMESTONE. 237 



We have here, I think, clearly the remains of considerable de- 

 posits which once occupied large areas in the valleys of the south 

 of Devon, and which, in part at least, are fairly entitled to be con- 

 sidered the lowland gravels of which Mr. Belt speaks. If they are 

 not, then in this immediate district these gravels are not repre- 

 sented. As to their date, there is substantial evidence that the same 

 stream, or streams, to which they are due, also carried into the 

 Oreston caverns the extinct mammals whose remains have been 

 found there, and that the cavern- and fissure- or ; « pocket "-deposits 

 are contemporaneous. 



Mr. Belt, following authorities of such eminence as Professor 

 Sedgwick and Sir H. De la Beche, and concurring with Mr. Joseph 

 Came, holds that the lowland gravels were spread out by a great 

 debacle. If this view be correct, the evidence should be continuous 

 throughout the district presumed to be exposed to its action. But, 

 in the deposits here briefly described, there is not only no evidence 

 whatever of a cataclysmal character, but every indication of orderly 

 deposition : — inland, nearer to the source of the debris, the bulk of 

 the pebbles and gravels proper ; further from their origin the sands 

 and clays, in fair orderly succession. 



The idea that the stanniferous gravels of Cornwall and Devon 

 were formed by some great catastrophe is at least three centuries 

 old ; for Carew, in his ' Survey,' says that the miners attributed 

 them to the deluge. Nearly fifty years since, Mr. Carne contended 

 that these gravels were deposited by a deluge of water from the north, 

 because the productive stream-works are situated in valleys opening 

 to the south. This fact of position, however, is easily explained by 

 reference to the peculiarity of the watershed of Cornwall. There 

 are only two rivers in the north of that county ; and one of these 

 does not pass through a stanniferous district. On the south coast 

 rivers abound. From its superior weight, the tin ore would 

 naturally form the lower layer of the deposits with which it was 

 associated — and this whether the whole of the materials were 

 in motion at once or not, since the effect of the continuous action 

 of a stream on gravel in its channel is to carry forward the lighter 

 and leave the heavier behind. Moreover the natural separation of 

 the tin-gravel would be materially aided by the occurrence of times 

 of flood. 



Sir Henry De la Beche clearly shows that all that is required to 

 account for the production of the stanniferous deposits is the exist- 

 ence of a mass of decomposed granite containing tin-ore, and its 

 subjection to the action of water — though it is not necessary, as I 

 hold, to assume with him that this water should be " driven vio- 

 lently against or over it." In such a body of decomposed granite I 

 conceive, not merely the stanniferous gravels of the Dartmoor rivers, 

 but the deposits of clay and sand here noticed, have their origin — 

 the clay representing the felspar, and the sand the quartz. This 

 accounts also for the absence of granitic pebbles in a deposit derived 

 from the Dartmoor Granite. 



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