FOLIATION OF CRYSTALLINE SCHISTS. 
595 
of volcanic rocks of the trachytic series, including some ob¬ 
sidians in Ascension, Mexico, and elsewhere, to their having 
moved when liquid in the direction of the laminae. The 
zones consist sometimes of layers of air-cells drawn out and 
lengthened in the supposed direction of the moving mass.^ 
Foliation of Crystalline Schists. —After studying, in 1835, 
the crystalline rocks of South America, Mr. Darwin proposed 
the foliation for the laminae or plates into which gneiss, 
mica-schist, and other crystalline rocks are divided. Cleav¬ 
age, he observes, may be applied to those divisional planes 
which render a rock fissile, although it may appear to the 
eye quite or nearly homogeneous. Foliation may be used 
for those alternating layers or plates of difierent mineralogic- 
al nature of which gneiss and other metamorphic schists are 
composed. 
That the planes of foliation of the crystalline schists in 
Norway accord very generally with those of original strati¬ 
fication is a conclusion long since espoused by Keilhau.f 
Numerous observations made by Mr. David Forbes in the 
same country (the best probably in Europe for studying 
such phenomena on a grand scale) confirm Keilhau’s opinion. 
In Scotland, also, Mr. D. Forbes has pointed out a striking 
case where the foliation is identical with the lines of stratifi¬ 
cation in rocks well seen near Crianlorich on the road to 
Tyndrum, about eight miles from Inverarnon, in Perthshire. 
There is in that locality a blue limestone foliated by the in¬ 
tercalation of small plates of white mica, so that the ro(ik is 
often scarcely distinguishable in aspect from gneiss or mica- 
schist. The stratification is shown by the large beds and col¬ 
ored bands of limestone all dipping, like the folia, at an an¬ 
gle of 32 degrees N.E.J In stratified formations of every 
age we see layers of siliceous sand with or without mica, al¬ 
ternating with clay, with fragments of shells or corals, or 
with seams of vegetable matter, and we should expect the 
mutual attraction of like particles to favor the crystalliza¬ 
tion of the quartz, or mica, or feldspar, or carbonate of lime, 
along the planes of original deposition, rather than in planes 
placed at angles of 20 or 40 degrees to those of stratification. 
We have seen how much the original planes of stratifica¬ 
tion may be interfered with or even obliterated by concre¬ 
tionary action in deposits still retaining their fossils, as in the 
case of the magnesian limestone (see p. 63). Hence we must 
expect to be frequently baffled'when we attempt to decide 
* Darwin, Volcanic Islands, pp. 69, 70. 
t Norske Mag. Naturvidsk., vol. i., p. 71. 
i Memoir read before the Geol. Soc. London, Jan. 31, 1855. 
