THE STRUCTURE OF ROCK MASSES. 
231 
of the mineral mostly lying, or, as it were, drawn out in one 
direction ; in fact, the metamorphism in this case appears to 
consist merely in a re-crystallisation of the same mineral pre- 
viously present in an amorphous or comminuted condition, 
without any true chemical action having’ necessarily been called 
into operation. When, however, instead of the beds being 
composed merely of one mineral, we find two of quite different 
nature — as, for example, mica and quartz — these two minerals, 
not being capable of reacting upon one another, except at such 
high temperatures as we have no reason to believe necessary 
for the development of foliation in rocks, usually segregate or 
separate from one another, so as to arrange themselves in more 
or less definite or distinct alternating layers, which, if the rock 
had been contorted or crumpled up by pressure, may often 
present the most fantastic appearances ; as, for example, in the 
mica schists in Anglesea, a fragment of which is depicted in 
PL LXXIII., fig. 8. 
Where one of the minerals has, for example, like garnet, 
a very great tendency to assume its crystalline form, we 
often find numbers of nearly, if not quite perfect dodecahedral 
crystals of this mineral, enveloped in the folise of mica or talc, 
which curve round them and compose the mass of the rock ; 
the crystals of those minerals which possess an elongated 
crystalline form, like andalusite, actinolite, kyanite, rhoetizite, 
&c., arrange themselves, as a rule, lengthways between the 
layers of mica, chlorite, &c. 
Many minerals which have less tendency to take the form 
of perfect crystals, appear, as it were, to segregate out in the 
form of lenticular or oval nodules, arranged with more or less 
regularity in the schist ; a very perfect example of this is seen 
in the annexed woodcut, which represents a fragment of di- 
Fig. i. 
chroite schist, consisting of a mass of mica (with a little talc), 
enclosing innumerable nodules of dichroite of a whitish or 
bluish-white colour, sometimes exhibiting the characteristic 
play of colours. This specimen is from the borders of Ong- 
steens Yand in southern Norway, where this rock extends over 
a considerable area, and as the nodules are pretty uniformly 
of about the size of a walnut, and the foliation of the mica 
binds itself around them, the rock itself presents a very pecu- 
