﻿Vol. 64.] HIGH-LEVEL PLATFOEMS OF BODMIN MOOR. 385 



The Middle Platform. 



The remains of the 750-foot platform are met with nearer the 

 sea, although still far up the valleys. This platform was first 

 recognized about Camelford, stretching from the granite-margin 

 westwards to the foot of Delabole Hill and sharply defined over 

 much of the area. 



Farther south, outside the western margin of the granite, 

 denudation has removed most of the platform, but it is again seen 

 within the granite-area about St. Breward, stretching for many miles 

 in a south-south-easterly direction. It clearly owes its preservation 

 to the superior hardness of the granite. 



On the northern and eastern side of the granite the 750-foot 

 platform is only encountered at a considerable distance seaward of 

 the edge of the 1000-foot platform ; but it is well marked about 

 Bowithick, where a large alluvial flat lies practically on it, at the 

 foot of the main slope formed by the edge of the granite. If the 

 granite-margin be followed south-eastwards, further traces of the 

 platform are met with in places, and they are specially well pre- 

 served where the rock in which the platform was cut consists of 

 epidiorite, a rock which, when fresh, has exceptional powers of re- 

 sisting denudation. Close to Trewint another extensive flat marsh 

 lies practically upon this platform, occurring likewise at the foot of a 

 great rising feature formed by the granite-margin ; but farther 

 south this feature commences below the 750-foot contour, and all 

 traces of the platform, here founded on killas, have been lost. 

 Above Trebartha Hall, this contour is seen to enter the granite 

 itself, well up this sharp slope, and here also a large alluvial flat 

 again indicates the position of the old platform. 



A strange phenomenon connected with both platforms is the 

 occurrence of extensive gently-sloping marshes at their higher 

 margin. Examples have already been given in the case of the 

 lower platforms, and the great Crowdy Marsh is a fine illustration 

 of one occurring on the upper platform. But they are not confined 

 to the higher platforms, for Mr. D. A. MacAlister and I found them 

 even better developed at the edge of the 430-foot or Pliocene 

 platform (Conce and lied Moors, south of Bodmin). 



Association of the Platforms with Superficial Deposits. 



An economic interest attaches to the 750-foot platform, as the 

 superficial deposits that bear tin, above this level, difi'er markedly 

 at times from those below it. It is evident that, when the sea 

 stood at the present 750-foot contour, there must have been 

 already much granite-detritus on the higher slopes, and this locally 

 contained tin-ore in abundance. But below this level there would 

 appear to have been little or none, for the country had either not 

 assumed the lower features which it at present possesses, or else the 

 tin-bearing deposit was swept off during emergence. Thus there is 



