THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 239 
in a schist are elongated parallel to the prism and that the elongation 
of the mineral corresponds generally with the direction of rock 
cleavage. In many cases the longer axes of the cross-sections of 
the hornblendes were observed to lie approximately in the plane 
of schistosity. According to Leith™ the ratio of the greatest to 
least diameters of hornblende, which in schistose rocks varies from 
100:20 to 100: 24, ranges in igneous rocks from 100:30 to 100:75 
with 100:40 as a common value. 
Feldspar possesses various cleavages but those parallel to the 
base and the side pinacoid are the most pronounced. ‘These two 
Fic. 3.—Biotites in schist. X32 Fic. 4.—‘ Porphyritic”’ biotites in 
schist. X32. 
cleavages are approximately at right angles to one another. Leith’ 
has stated that in schistose rocks the parallelism of the feldspar 
particles is not close and that rarely the parallel arrangement is 
also crystallographic. The writer’s observations on similar rocks 
indicate that feldspar grains are occasionally elongated in the 
direction of rock cleavage but no relation was noticed between the 
elongation and the crystallographic directions of the grains. 
Conclusions regarding the causes of mineral elongation.—It has 
been noted that under certain conditions minerals tend to assume 
an abnormal elongated habit. High viscosity or strong super- 
1 Op. cit., 30-31. 
2 Op. cit., 40. 
