THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 233 
from a fluid and in the other recrystallization of a solid under 
differential pressure. Grubenmann considers that the chief expres- 
sion of these different conditions of crystallization is to be found in 
the forms or outlines of the mineral constituents. 
Unusual development of cleavage faces with consequent pro- 
duction of columnar or platy mineral forms is, according to Gruben- 
mann, one of the most common characteristics of the minerals of 
‘crystalline schists.”” Wishing to make use of this feature in a 
discussion of the texture of primary gneisses, the writer will review 
the causes of the unusual elongated habit assumed by minerals 
when growing under certain conditions. No mineral seems to 
exhibit better this capacity for abnormal form development than 
biotite which, happily also, is one of the most characteristic minerals 
of primary gneisses. The writer in the following discussion 
expresses the opinion that the biotite grains in normal igneous 
rocks are roughly equidimensional in shape, and that the platy 
forms present in metamorphic rocks and in primary gneisses are 
the result of crystallization under differential pressure. This 
would seem to suggest that the texture of primary gneisses must be 
intermediate between the igneous and the metamorphic types. 
Microscopic evidence seems to lead to the same conclusion. The 
writer, however, wishes to point out that from the very character 
of the intrusion of primary gneisses it is to be expected that granu- 
lation and recrystallization have frequently taken place after 
solidification and that, accordingly, a metamorphic texture cannot 
be regarded as proof that the banding in a gneiss was not produced 
when the rock mass was still partially fluid. It is the writer’s view, 
however, that with certain limitations, igneous texture, when 
present, may be legitimately urged as proof of primary banding. 
DISTINCTION BETWEEN IGNEOUS AND METAMORPHIC TEXTURES 
By the term “texture” the writer understands the character 
of a thin section or surface of a rock due to its degree of crystallinity 
and to the size, shape, and arrangement of its minerals. 
Crystalloblastic texture—The term “‘crystalloblastic’’* has been 
proposed as a designation for the texture of recrystallized rocks. 
«F, Becke, Tschermaks Min. petrog. Mitt., XXI (1902), 356-57. 
IIT 
