92 Remarks on the Transition Rocks of Werner. 
jts aspect is tame in the extreme, being comparatively flat,— 
‘a circumstance visibly occasioned by the corroding opera- 
tions of time. Nowhere are the vestiges of degradation so 
remarkable as bere. The enormous deposites of tin in the 
different stream-works, of which that of Carnon is perhaps 
the most extensive, clearly prove the destrnction of sur- 
rounding motntains. 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 
overburthen of forty feet thick, and occupying a valley of 
very great extent. The lodes which furnished this tin 
must have existed above the Jevel of the deposite; and 
from the quantity of metal deposited, they must have oc- 
cupied a large tract of country. Other monuments of tbis 
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 decomposi- 
tion, and are formed of huge masses of rock, apparently 
piled on each other, with a regularity resembling masonry, 
and in all respects similar to the arrangement observable on 
ahe summit of every mountain in Arran, where the traces 
of time are also deeply furrowed. 
Roach Rock, a binary compound of quartz and horn- 
blend, is another very remarkable instance of the same 
fact: this rock is flat at the top, and being quite perpendi- 
cular 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 church. Phere can be 
no doubt that this singular rock owes its present appearance 
to the operations of time on the surrounding materials, 
which its peculiar composition has enabled it to with- 
stand, 
The killas likewise presents marks of degradation, where 
the country is composed of that rock. I noticed in some 
districts the roads mended entirely with quartz, (No. 24 )3 
the brilliant white appearance of which, after a shower, had 
avery curious effect. I could not comprehend by what 
industry the accumulated heaps.of this substance were ob- 
tained: at last I perceived that they were gathered from 
the adjoining fields, and in some places picked from the 
surface of a common, by means of a hoe or mattock. 
That fragments of quartz should occur so unmixed with 
any others, is only to be accounted for by supposing that 
they formed the quartz veins in the killas, which, from 
superior tenacity, resisted decomposition, while the softer 
: parts 
