T. BELT ON THE DRIFT OF DEVON AND CORNWALL. 



81 



Fig. 1. — Diagram Section of Excavation made in search of Stream-tin. 



Excavation. 



a. Soft decomposed granite, b. Stratified granite sand. c. Gravels and sand 

 with boulders, d. Large blocks of granite lying on the surface. 



Many other sections in the old stream-tin workings and in the 

 banks of rivulets show a similar succession, If the blocks of granite 

 had been left by the decomposition of softer portions of the bed-rock, 

 they would have rested upon it. Neither can they have slipped 

 down from the " tors " that crown many of the hills ; for they are 

 not confined to the slopes, but are spread out over the level ground 

 as well, neither are they concentrated at the bottom of slopes, 

 as they would have been by such an action. Here and there two 

 or three, or even a dozen may be crowded together ; but generally 

 they are separated from each other. 



Near to Okehampton, on the northern'part of the moors, the bed- 

 rock is composed of hardened shales and sandstones. The sides of 

 the hills are covered with a stony clay, sometimes as much as 30 

 feet in thickness, containing many large angular blocks of stone, 

 especially near to, and on, the surface. Amongst these, blocks of 

 granite, that must have been brought at least two miles, are not 

 unfrequent. 



On the eastern side of Dartmoor, Mr. G. W. Ormerod has noticed 

 beds of gravel reaching up to about 900 feet above the sea, and 

 has given instances of many transported blocks of granite and Car- 

 boniferous rocks. Thus, to the east of Cranbrook Castle (1110 feet 

 above the sea) and near "Wooston Castle, large transported granitic 

 blocks overlie the Carboniferous beds. On the Newton and Moreton- 

 Hampstead Railway, fragments of granite are spread over the Car- 

 boniferous strata ; and gravel formed of elvanic and Carboniferous 

 rocks occurs at Kiddy Hill, its nearest point of derivation being on 

 the opposite side of the valley *. Mr. George Maw has described 

 the occurrence at Petroclistow, near the centre of the county, of an 

 isolated bed of gravel, composed almost entirely of the detritus of 

 Dartmoor granite, twelve miles distant from the nearest granitic 

 mass, and considers that it can only be accounted for by an amount 

 of submergence covering the whole of the ridges f. 



In one of the many able communications Mr. D. Mackintosh has 

 made on the Glacial beds, he has advanced arguments to prove that 



* Quart. Journ. Geol. Soc. vol. xxiii. p. 418. 

 Q. J. G. S. No. 125. 



t Ibid. vol. xx. p. 451. 



