238 J. D. TRUEMAN 
ratios were sometimes lower than this. An average of 46 observa- 
tions on biotites of the Wissahickon’ sedimentary gneiss gave a 
value of 7.2. In all schistose rocks so far mentioned the direction 
of mineral cleavage and elongation corresponded, in general, with 
the plane of rock cleavage. When biotites crystallize under mass- 
static conditions, i.e., in the absence of differential pressure, the 
form of the biotite grains, as shown in Fig. 4, seems to approach 
that characteristic of normal igneous rocks. In such cases there 
is commonly no relation between the direction of mineral elonga- 
tion and that of rock cleavage. These “porphyritic”’ biotites may 
Fic. 1.—Biotites in nepheline syenite. X32 Fic. 2.—Biotites in primary gneiss. 32 
frequently be recognized by the number of inclusions they contain, 
a feature which is not common in grains which have crystallized 
under differential pressure. From the foregoing observations it 
may be seen that a platy form is not always characteristic of 
biotite, as is sometimes supposed, but that this habit is determined 
by the conditions of its formation. Such observations as the 
writer has made indicate that the effect of differential pressure on 
the forms of sericite and chlorite is analogous to that in the case 
of biotite. 
Hornblende possesses a prismatic cleavage, the angle between the 
planes being about 124°. Leith? has shown that the hornblendes 
t U.S.G.S. Folio 162, 1909. 
20 Ui. S).GASe Datll 23020). 
