312 J. D. TRUEMAN 
may be divided into two classes: (1) those which received their 
foliation during consolidation from an igneous melt, (2) those 
in which the foliation is a secondary structure. Many geologists 
do not acknowledge the importance of the first group though 
gneisses have been described as such by many well-known writers. 
A review of the criteria for distinguishing these primary gneisses 
from metamorphic foliated rocks shows that the most significant 
distinctions are based on field observations. It has been fre- 
quently considered important to determine whether certain foliated 
crystalline rocks were originally sedimentary or igneous in char- 
acter. Numerous field, microscopic, and chemical methods have 
been suggested for distinguishing these classes but it can hardly 
be claimed that the results of their application have been entirely 
satisfactory. The various methods proposed have been reviewed 
by the writer. 
Three of the methods suggested for the identification of foliated 
crystalline rocks have been treated in the preceding article in some 
detail. They are: (1) the criterion of texture as applied to primary 
gneisses, (2) uses of zircon as a criterion (chiefly for distinguishing 
the original igneous or sedimentary character of rocks), (3) use 
of chemical composition as a means of determining igneous or 
sedimentary origin. 
It seems plausible that primary gneisses, which are in reality 
only igneous rocks with a banded structure, could be distin- 
guished from metamorphic rocks by means of texture. Gruben- 
mann has made a careful study of the texture of metamorphic 
rocks and has proposed a rather complete system of nomenclature 
for the different types. This is being adopted by many German 
geologists but its introduction into English is rendered difficult by 
the conflicting meanings of ‘‘texture’’ and “structure” in the two 
languages. Grubenmann calls the texture characteristic of ‘crys- 
talline schists” ‘‘crystalloblastic”’ and considers that it is distin- 
guished from igneous texture largely by features caused by the 
simultaneous crystallization of the different minerals instead of 
the more or less successive crystallization common to igneous 
rocks. Milch, according to a recent article, seems of the opinion 
that texture will be found to be the distinguishing feature of 
