clay slate near the ferry to Kerrera, a few miles south of Oban, in
Argyleshire. This bed has been partly wasted away by the sea, and
its structure exposed to view. It contains a central cylindrical
nucleus of unknown length (but certainly considerable), round which
six beds of clay slate are wrapt, the one within the other, so as to
form six concentric cylinders. Now, however plastic the clay slate
may have been, there is no kind of pressure which will account for
this structure; the central cylinder would have required to have
been rolled six times in succession (allowing an interval for
solidification between each) in the plastic clay slate."--_Outlines
of Mineralogy, Geology, &c._, by Thomas Thomson, M.D.

CHAPTER X.

OF THE MATERIALS OF MOUNTAINS:--THIRDLY, SLATY COHERENTS.

Sec. 1. It will be remembered that we resolved to give generally the term
"coherent" to those rocks which appeared to be composed of one compact
substance, not of several materials. But, as in all the arrangements of
Nature we find that her several classes pass into each other by
imperceptible gradations, and that there is no ruling of red lines
between one and the other, we need not suppose that we shall find any
plainly distinguishable limit between the crystalline and coherent
rocks. Sometimes, indeed, a very distinctly marked crystalline will be
joined by a coherent rock so sharply and neatly that it is possible to
break off specimens, no larger than a walnut, containing portions of
each; but far more frequently the transition from one to the other is
effected gradually; or, if not, there exist, at any rate, in other
places intervening, a series of rocks which possess an imperfectly
crystalline character, passing down into that of simple coherence. This
transition is usually effected through the different kinds of slate; the
slaty crystallines becoming more and more fine in texture, until at last
they appear composed of nothing but very fine mica or chlorite; and this
mass of micaceous substance becomes more and more compact and silky in
texture, losing its magnesia, and containing more of the earth which
forms the substance of clay, until at last it assumes the familiar
appearance of roofing-slate, the noblest example of the coherent rocks.
I call it the noblest, as being the nearest to the crystallines, and
possessing much in common with them. Connected with this well-known
substance are e