iS13.] 



Transition Rocks of JVerner. 



422 



quartz, and mica, other fossils sometimes occur io it 3 ofthese^ 

 bchorl is the most frequent, then garnet and tinstone.^ 



At Penzance I observed some buildings constructed of a re- 

 markably fine-grained granite; but this I no where saw i?i situ: 

 otherwise, from Teign Bridge, where I first set my foot on. 

 granite, to the Land's End, it is generally of that character 

 which entitles it to be ranked with the oldest variety. In many 

 places it has suffered to a most wonderful extent by decomposi- 

 tion, but where it retains its freshness, no granite can possibly 

 be better characterised. The specimens which I v/as able to 

 bring away, and which are now before the Society, are by no 

 means adequate to convey an idea of the coarse texture it some- 

 times presents. In the granite of Dartmoor the crystals of 

 felspar are uncommonly large, often four inches in length. I 

 believe it was from this neighbourhood that the flags of the foot- 

 path on Westminster 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 Corn- 

 wall is a most decided exception : its aspect is tame in the 

 extreme, being comparatively flat, — a circumstance visibly occa- 

 sioned by the corroding operations of time. Nowhere are the 

 vestiges of degradation so remarkable as here. The enormous 

 deposites of tin in the different stream-works, of which that of 

 Carnon is perhaps the most extensive, clearly prove the destruc- 

 tion of surrounding mountains. This tin, in the shape of 

 rounded pebbles, formed a stratum of about a foot thick, under 

 a deposite of granite -gravel and mud, together forming an over- 

 burthen of 40 feet tljck, 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 v;hich are seen in every direction in the granite dis- 

 tricts of Cornwall, These are evidently the result of surrounding 

 decomposition, and are formed of huge masses of rock, appa-^ 

 rently piled on each other, with a regularity resembling masonry, 

 and in all respects similar to the arrangement observable on the 

 summit of every mountain in Arran, v^iiere the traces of time 

 are also deeply furrowed. 



Roach rock, a binary compound of quartz and hornblende, h 

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

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

 when viewed from the west, presents the appearance of a square 

 castellated building, which is rendered more conspicuous by 

 being nearly of the same height as the tower of an adjoining 



* Jameson's Mineralogy, vol. Wh p. 102, &Co 



