Mr. Sharpe, I believe, attributes the
cleavage to the mere mechanical pressure of enormous weights of rock,
especially where crushed by earthquakes. Professor Rogers, again,
points out that as these slates may have been highly heated, thermal
electricity (i.e. electricity brought out by heat) may have acted on
them.
One thing at least is clear. That the best slates are found among
ancient lavas, and also in rocks which are faulted and tilted
enormously, all which could not have happened without a
proportionately enormous pressure, and therefore heat; and next, that
the best slates are invariably found in the oldest beds--that is, in
the beds which have had most time to endure the changes, whether
mechanical or chemical, which have made the earth's surface what we
see it now.
Another startling fact the section of Snowdonia, and I believe of
most mountain chains in these islands, would prove--namely, that the
contour of the earth's surface, as we see it now, depends very
little, certainly in mountains composed of these elder rocks upon the
lie of the strata, or beds, but has been carved out by great forces,
long after those beds were not only laid down and hardened, but
faulted and tilted on end. Snowdon itself is so remarkable an
instance of this fact that, as it is a mountain which every one in
these happy days of excursion-trains and steamers either has seen or
can see, I must say a few more words about it.
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