ORIGIN OF FOLIATED CRYSTALLINE ROCKS 213 
primary gneisses. Views regarding the mode of formation and the 
shape of certain minerals of primary gneisses have been discussed 
by the writer in order that the manner of the crystallization of 
these rocks may be understood. It has frequently been stated that 
the foliation in such rocks is due to the rotation of minerals in a 
still fluid magma. While this, no doubt, occurs in some instances 
the writer has endeavored to show largely by measurements of 
biotite grains that elongation of mineral constituents by crystal- 
lization under differential pressure must also be a very important 
factor in producing foliation. It is concluded that the typical 
texture of primary gneisses is more or less intermediate between the 
igneous and the metamorphic types but that owing to the condi- 
tions of formation of these rocks a crystalloblastic or even cata- 
clastic texture may be superimposed on the original. Texture as 
a means of identifying primary gneisses seems, accordingly, of 
only limited application. 
The suggestion that zircon be used as a criterion for the identi- 
fication of igneous or sedimentary origin was made by Derby in 
1891, but the method has not been adopted by many geologists. 
Zircons are widely distributed in igneous rocks and during sedimen- 
tation become rounded in form and concentrated in the arenaceous 
deposits. It is proposed that their presence and their character 
may serve as a means for determining the original character 
of foliated rocks. Essential to the application of zircon as a cri- 
terion is the question of its stability under metamorphic conditions. 
Derby was of the opinion that zircon could not form in a rock 
secondarily. The writer has shown by an example of secondary 
enlargement of zircon grains that this is not impossible. Other 
cases, however, appear to prove that zircon is sufficiently stable 
to be used as a criterion. The conclusions which seem justified 
regarding the use of zircon in this connection are briefly as follows: 
abundant, minute zircons in a rock indicates that the original 
rock was either igneous or an arenaceous sediment; when the 
grains are uniform in character, well crystallized, and fresh in 
appearance an igneous rock seems likely; when they are well 
rounded and lacking in luster the original rock was probably 
sedimentary; absence of zircon grains is confirmatory of sedi- 
