TRANSITION ROCKS. 123 



The specimens which I was able to bring away, and which 

 are now before the Society, are by no means adequate to con- 

 vey an idea of the coarse texture it sometimes presents. In 

 the granite of Dartmoor, the crystals of felspar are uncommon- 

 ly large, often four inches in length. I believe it was from 

 this neighbourhood that the flags of the footpath on Westmin- 

 ster Bridge were brought ; in these, crystals of felspar nearly as 

 large may be observed. 



Granite countries usually present a bold and varied outline ; 

 but to this rule Cornwall is a most decided exception : its as- 

 pect is tame in the extreme, being comparatively flat, — a cir- 

 cumstance visibly occasioned by the corroding operations of 

 time. Nowhere are the vestiges of degradation so remark- 

 able as here. The enormous deposites of tin in the different 

 stream-works, of which that of Carnon is perhaps the most ex- 

 tensive, clearly prove the destruction of surrounding moun- 

 tains. This tin, in the shape of rounded pebbles, formed a 

 stratum, of about a foot thick, under a deposite of granite-gra- 

 vel and mud, together forming an overburthen of forty feet 

 thick, and occupying a valley of very great extent. The lodes 

 which furnished this tin must have existed above the level of the 

 deposite ; and from the quantity of metal deposited, they must 

 have occupied a large tract of country. Other monuments of this 

 general destruction may be found in the peaks which are seen 

 in every direction in the granite districts of Cornwall. These 

 are evidently the result of surrounding decomposition, and are 

 formed of huge masses of rock, apparently piled on each other, 

 with a regularity resembling masonry, and in all respects simi- 

 lar to the arrangement observable on the summit of every 

 mountain in Arran, where the traces of time are also deeply 

 furrowed. 



Roach Rock, a binary compound of quartz and hornblend, 

 is another very remarkable instance of the same fact : this rock 

 is flat at the top, and being quite perpendicular on three sides, 



Q 2 when 



