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but rather as the conclusions arrived at by the author, after a 
prolonged study of this subject in the field and in the laboratory. 
Although, as before mentioned, all foliated rocks are meta- 
morphic rocks, these may have originally been either of sedi- 
mentary or eruptive nature ; for a long time it was supposed 
that foliated structure was alone characteristic of the so-called 
crystalline schists, until further inquiry into the subject showed 
that, besides being common in the crystalline or so-called 
primitive limestones, it was far from being of rare occurrence 
in many of the plutonic and volcanic eruptive rocks, abundant 
examples being met with of foliated granites, syenites, gabbros, 
trachytes, lavas, &c., in which a distinctly recognisable parallel 
structure is developed by the manner in which one or more of 
the crystalline mineral components are disposed in the mass of 
the rock. 
The parallelism of foliation is not, as in the case of the 
other structure already described (stratification, . joints, and 
cleavage), brought about by the formation of divisional planes 
due to the effects of purely mechanical forces, but, on the 
contrary, is invariably determined by the presence of crystal- 
lised minerals, usually in alternating layers, very different from 
one another both mineralogically and chemically, and which, 
owing to the peculiar nature (habit, or behaviour, as it has 
been called by mineralogists) of the minerals themselves, and 
to the pressure to which they have been subjected, when in 
the process of formation or crystallisation, by the weight of the 
superincumbent mass above them, most commonly assume the 
form of crystallised foliae, or of crystals developed mainly in 
the direction of one of their axes only. From this it will be 
seen that it is very easy to discriminate between foliation and 
all other parallel structures likely to be encountered in rock 
masses. If at times (as in the coal formation, for example) we 
find minor sedimentary beds, made up almost entirely of plates 
of mica, or sandstones possessing a fissile or laminated structure 
from numerous scales of mica which may be arranged in 
more or less parallel lines, a closer examination of the rock, 
and more particularly of the mica in it — using the microscope 
if necessary — will at once show, that it has been deposited as a 
sediment from water, as the particles will be found waterworn 
and abraded, and to present an appearance totally different 
from that of the foliae, crystallised in situ , which are met with 
in the crystalline schists or other true foliated rocks. 
The simplest form of foliated rocks which occur in nature 
are those beds of crystalline schist, solely composed of one 
mineral, such as many of the mica, chlorite, talc, or hornblende 
schists ; in these, as shown in PL LXXIII., fig. 4, the rock is 
seen to be a mere aggregation of imperfectly developed crystals 
