232 J. D. TRUEMAN 
mination of sedimentary or igneous origin. It must be remem- 
bered, however, that one type of criterion can seldom be employed 
effectively alone, although the limitation of possibilities obtained 
by the application of several criteria may lead to evidence that is 
practically conclusive. 
The writer is deeply indebted to Dr. C. K. Leith and other mem- 
bers of the geological department of the University of Wisconsin 
for assistance and suggestions received during the preparation of 
this article. 
TEXTURE AS A CRITERION FOR THE IDENTIFICATION OF 
PRIMARY GNEISSES 
OUTLINE OF DISCUSSION 
In order that the reader may more easily understand the trend 
of the argument which the writer will advance regarding the value 
of texture as a criterion for the identification of primary gneisses, 
the discussion which follows is here summarized. 
Milch™ in a recent article expresses the opinion that igneous 
rocks with an original foliation should not be included in the group 
of the ‘‘crystalline schists.” He regards texture as the most 
promising criterion so far brought forward for distinguishing these 
rock classes. It is the suggestion that texture may be used as a 
criterion for distinguishing primary gneisses from those of meta- 
morphic origin which the writer proposes to examine in the course 
of the present paper. Milch’s idea was that “crystalline schists”’ 
are characterized by metamorphic or ‘‘crystalloblastic”’ texture, 
while gneissic rocks which possess an original foliation have the 
texture of igneous rocks. In order to get a clearer conception of 
the differences between these two varieties of texture, the writer 
will review the main features of metamorphic texture according 
to Grubenmann,? whose recent work marks a decided advance in 
the study of that subject. The causes underlying the differences 
between these two types of texture are apparently to be found in 
variations in conditions of crystallization; in one case solidification 
tL. Milch, “‘Die heutigen Ansichten tiber Wesen und Hnstenung der kristallinen 
Schiefer,”’ Geol. Rundschau, I (1910), 49. 
2U. Grubenmann, Die kristallinen Schiefer, I (1904), II (1907). 
