﻿386 ME. G. EAEEOW ON THE [Aug. I908, 



a far greater amount of decomposed material, capable of supplying 

 stream-tin, above this critical level, than below it ; because above 

 this level we find the results of decomposition occurring both before 

 and after the 750-foot platform was cut, while below it the results 

 of the later decomposition alone are found. 



Association with Stream-Tin. 



The great extent of the stream-works in the higher parts of 

 Eodmin Moor must be attributed to this cause. In strips of 

 varying width they cover mile after mile of country, and the 

 industry for 1600 years has been carried on in situations that have 

 no parallel farther south in this district or at lower levels. The 

 decomposition-products of the granite are to be seen wherever they 

 could find lodgment, and in many cases they contained much tin- 

 ore. The most interesting occurrences are those met with on the 

 cols of watersheds, in hollows between two larger hills, a situation 

 in which, farther south, we should not expect to find either 

 decomposition-products or wash in any considerable quantity. And 

 yet, in one instance, a thickness of no less than 40 feet has been 

 proved (see PI. XLYI) ; and the antiquity of the deposit is so great 

 that every trace of felspar-crystals has vanished, leaving a yellow 

 clay in which even the quartz-crystals have fallen to pieces. 



Similar material in similar positions has been met with all over 

 the higher part of the granite of Bodmin Moor, and is especially 

 abundant in the more northerly portion, where it has often been 

 found to contain a considerable amount of stream-tin. The fact 

 that these older products of decomposition of the granite (wash or 

 head) are preserved only at high altitudes and at a considerable 

 distance inland from the coast, suggests at once a probable 

 explanation of their preservation, namely : that these soft deposits 

 must have been either frozen, or buried under a small snow-field 

 that accumulated during a part of the Glacial Epoch, when the 

 country farther south was subject to violent floods and that farther 

 north was more or less covered by an ice-sheet. The material 

 itself is, of course, in parts of the same age as the 'head' of the coast, 

 generally held to have been formed during colder conditions, and 

 actually has a distinct Glacial deposit associated with its upper part 

 in the Scilly Isles. The large district, much of it at least 1000 feet 

 above sea-level, on the southern edge of which these loosely-coherent 

 deposits have been somehow protected from denudation, is to this 

 day the coldest part of Cornwall ; so it would of necessity have been 

 colder than the coast-area, in which signs of a colder climate are so 

 clear. Indeed, actual evidence of the snow-field exists in the radial 

 distribution of the great boulders of granite from Eoughtor and 

 similar high crags, suggesting that the rocks slid down snow-slopes, 

 an explanation tentatively offered by Mr. Clement Eeid. 



"Whatever explanation is finally adopted to account for the 

 preservation of the old wash on the hill-slopes, this material is 

 fundamentally different from the normal valley-deposits, in which 



