File:The Quarterly journal of the Geological Society of London (12711862804).jpg

170
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Jail. 31,
sometimes foliated by the chlorite, so as to resemble a green mineral.
The annexed diagram, fig. 2, will give an idea of these conditions.
Fig. 2. — Quartz-vein in clay-slate at Luss, Loch Lomond^ Scotland.
In general in foliated rocks there is a totally
altered structure, and we have many instances
where they hardly possess any character in
common with cleaved rocks.
Thus, even the parallelism of the lines cannot
be considered invariable ; we certainly have
some cases where the foliation coincides vrith
the bedding and not with the cleavage, and
the property of splitting into laminae in certain
directions is often not possessed by foliated
rocks at all, particularly if the foliating mineral
be not itself of laminar structure.
We find certainlv that in considerable di-
stricts of country the cleaved rocks are totally
\ different from those in another district, just as
one formation might be expected to differ from
another ; but it is doubtful if we ever find
such abrupt and total changes as come in such
rapid succession in the beds of foliated rocks.
(Case 6.) — In a quarry on the roadside about two miles from
Crianlorich, in Perthshire, I found the strike N.W.-S.E., with dip
30° N.E. ; the rock consisted of very thin beds which alternately pre-
sented the character of perfect and highly micaceous mica-schist and
of extremely quartzy schist, so that it was not possible to call one
single linear foot by the same name. Other cases more unusual will
be noticed in the course of the communication.
The production of foliated structure is not confined to the intro-
duction of laminae of one or two mineral substances, as mica, horn-
blende, chlorite, &c. ; it may be produced by minerals widely dif-
ferent in chemical composition and mineralogical character, and the
presence of which is only to be accounted for by the supposition that
the constituent elements must have been at hand in the unmetamor-
phosed rocks, although in a different state of combination.
It is apparently also a necessary requisite in the production of
foliated arrangement, that the minerals thus formed be of a different
chemical constitution to those composing the mass of the rock itself.
I may here bring forward some instances of very distinct and de-
terminate foliation produced by minerals not usually found under
these circumstances.
(Case 7.) — On the heights immediately above Christiansand, in
Norway, probably at a distance of two miles from the town, the
section represented by fig. 3 was taken. The prevailing rock is here
gneiss (a «), composed of black mica, white quartz, and white and
red felspar, with sometimes specks of black oxide of iron. The