It is in the solid parts of those
strata, perfectly disconnected from the veins, that there are frequent
cavities curiously lined with crystals of different sorts, generally
calcareous, sometimes containing also those that are siliceous, and
often accompanied with pyrites. I am persuaded that the origin of those
cavities may have been some hollow shells, such as _echini_ or some
marine object; but that calcareous body has been so changed, that it is
not now distinguishable; therefore, at present, I hold this opinion only
as conjecture.
Having, in my return to Edinburgh, traveled up the Tiviot, with a view
to investigate this subject of primary and secondary operations of the
earth, I found the vertical strata, or alpine schistus, in the bed of
the river about two miles below Hawick. This was the third time I had
seen those vertical bodies after leaving the mountains of Lauderdale.
The first place was the bed of the river Tweed, at the new bridge below
Melrose; but here no other covering is to be seen above those vertical
strata besides the soil or traveled earth which conceals every thing
except the rock in the bed of the river.
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