THE INSULATION OF ST. MICHAEL'S MOUNT. 23 
been made and recorded respecting the stream tin works in Corn- 
wall, we have information on the question of a most trustworthy 
character, and well calculated to impress the mind with the vast- 
ness of the time which has elapsed since the subsidence. Thus, 
at Carnon, Mr. Henwood found the vegetable bed resting on the 
“tin-eround,” and.lying beneath a series of distinct beds of sand 
and silt, having a total thickness of more than 43 feet, and all of 
them, with the exception of the uppermost bed of river sand and 
mud, three feet in thickness, containing marine shells.* 
Again, Mr. Colenso found at Pentuan the old forest rooted in 
the “tin-ground,” and overlaid with detrital matter 64 feet in 
ageregate thickness. This accumulation, too, was made up of dis- 
tinct beds of sand, silt, and vegetable matter; every portion of 
which had been deposited since the subsidence, for an oyster bed 
was found on the top of the ‘“tin-gronnd,” the shells being still 
fastened to some of the large stones and the stumps of the trees.} 
When it is remembered that beds of sedimentary origin, like 
those just mentioned, can no more be deposited at a rate exceeding 
that at which the pre-existing rocks are abraded, than a wall can, 
on the whole, be built faster than the stones are quarried or the 
clay is dug for making the bricks, it will be felt to be impossible 
to regard the strata under consideration as representing a few 
centuries merely; and, though we are not, and never may be, able 
to evaluate them in years, we cannot but feel that they bring us 
face to face with an enormous amount of time. 
Should it be objected that a deposit of great thickness may, 
by a change in the direction or velocity of a stream, be removed. 
from one place to another and re-deposited in a comparatively 
short time, it may be replied that the objection is in itself un- 
doubtedly valid, but that it does not, and cannot, apply to the 
cases before us, as, in each, the successive beds were perfectly 
distinct and of dissimilar materials, and, at. all levels, contained 
marine shells, which, and particularly the flat ones, Mr. Colenso 
states were frequently found in rows or layers. They were often 
double and closed, with their opening part upwards, as if the fish 
had lived and died where their remains were found. 
* Trans. Roy. Geol. Soc. of Cornwall, Vol. iv., p. 57, et. seq., 1829. 
+ Ibid, p. 29, e¢ seq. 
t Op. cit. 
