THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 227 
by a parallel arrangement of mineral constituents is, of course, a 
fact which is accepted by all writers. Perhaps, however, it is not 
so generally understood that the shapes of the so-called “platy 
minerals’ of these. rocks are generally different from those which 
the same minerals assume when growing under freer conditions. 
To illustrate this difference in shape, the writer will discuss the 
dimensions of quartz, biotite, hornblende, and feldspar in igneous 
and metamorphic rocks. These minerals were selected because 
they vary in the character of their cleavage and all are, moreover, 
common rock-forming constituents. 
Quartz is the most common example of a group of minerals 
which possess no good cleavage. In metamorphic rocks it owes 
its form in the majority of cases either to granulation or recrystalli- 
zation. When the grains result from granulation they are generally 
irregular in shape and roughly equidimensional, but the writer’s 
observations seem to show that those grains which have crystallized‘ 
under differential pressure are frequently elongated parallel to the 
plane of rock cleavage. It appears, however, that the ratio of the 
dimension of the quartz grain in the direction of schistosity to that 
at right angles to it is seldom greater than two. Generally, also, 
as Leith? has pointed out, there is no relation between the elongation 
of quartz and its crystallographic directions. 
Biotite is a mineral which is characterized by a single well- 
marked cleavage. Chlorite and sericite, both important rock- 
forming minerals, are similar to biotite in respect to cleavage. In 
the following measurements of biotite grains the ratio of the length 
in the direction of mineral cleavage to that across it has been deter- 
mined. An average of 167 grains of biotite in 19 sections of igneous 
rocks showed a ratio of 1.5, but many soda-rich rocks gave average 
values of about 1.0, and in rocks with porphyritic texture the ratio 
rose sometimes to 2.0 or even 2.5. In the latter case the rocks 
have probably crystallized quickly and under somewhat viscous 
conditions. Similar measurements of biotites in sections of schists 
generally gave average values above 6, though when the develop- 
ment of schistosity had taken place near an igneous contact the 
1 The criteria for recognizing recrystallized minerals has been discussed by Leith 
in U.S.G.S. Bull. 239 (1905), 70-71. 
2U.S.G.S. Bull. 239, 35- 
