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402
PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
.May 6,
laminar trachytes, or to some little-understood force of polarity, it
appears to me far from superfluous to describe certain facts very
completely establishing the sedimentary and metamorphic origin of
the schistose rocks of the localities mentioned in the sequel. That
the mere repetition of layers of different mineral character is of
itself no absolute proof of sedimentary formation is sufficiently
apparent from what is often seen in well-known igneous rocks ; but
mica-schist sometimes possesses other peculiarities, to which I beg-
to call attention.
In very many stratified rocks the ripple-marks are extremely
characteristic of deposition under the influence of a current ; but, if
these were found in bent and contorted schists, it would, I think,
be rash to base any important conclusion on them, since mere
mechanical bendings might so readily mislead an observer. There
is, however, a structure generated when ripples are formed whilst
material is being deposited and permanently accumulated, which
has such well-defined peculiarities that it appears to me almost
impossible to confound it with any structure produced by other
means. I here allude to what, in the various papers I have pub-
lished on the subject*, I have called " ripple-drift," the most im-
portant characters of which will be better understood from the fol-
lowing drawing : —
Fig. 1. — Tliin bed of "Ripple- drift " in unaltered sandstone (nat.size).
Frequently there are quite horizontal beds below and above
(a b, cd), and between are beds inclined to them at an angle a dc y
which are themselves made up of smaller beds or stratula dipping in
the opposite direction, namely from b towards c. Such bands often
occur in the green slates of Langdale in Westmoreland, the cleavage
cutting them at a considerable angle ; and when the slates arc
ground smooth and slightly varnished, the peculiar structure may

• Eilin. New Phil. Jburn., new ser. vols. iii. p. 112, iv. p. 317, v. p. 275,