THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 229 
PAR 
INTRODUCTION 
REVIEW OF CRITERIA PROPOSED FOR THE DETERMINATION OF THE 
ORIGIN OF FOLIATED CRYSTALLINE ROCKS 
Criteria constitute one of the most important divisions of a 
geologist’s working ‘‘equipment.’’ While they are needed and 
are being developed along the whole line of attack on the problems 
of geology, probably no section of investigators appreciates their 
value more than that engaged in unraveling the history of foliated 
_crystalline rocks. This seems to be because that subject is par- 
ticularly many-sided and difficult, for it cannot be said that sug- 
gestions as to methods of approach are at all lacking in number. 
To indicate the range of these proposals, and to give an idea of the 
state in which the problem stands today, the better known criteria 
for the determination of the origin of foliated rocks have been 
collected in the lists which follow. It must be acknowledged, 
however, that in many cases the results of their application are 
more suggestive than conclusive. 
Criteria for the determination of original igneous or sedimentary 
character.—The following have been suggested as criteria for dis- 
tinguishing foliated rocks which were origmally sedimentary from 
those developed from igneous rocks: 
Field evidence: For igneous origin—gradation into recognizable 
igneous rocks; preservation of original structures, such as bound- 
aries of a dike; preservation of original textures, such as por- 
phyritic; uniformity over large areas. For sedimentary origin— 
gradation into normal sedimentary rocks; preservation of original 
structures, such as pebbles of a conglomerate or cross-bedding; 
regular and continuous banding;? intercalation with beds of lime- 
stone or quartzite;? rusty weathering. 
Microscopic evidence: For igneous origin—preservation of 
igneous textures in the less altered portions; presence of minerals 
characteristically formed only from igneous melts and readily 
tJ. F. Kemp, “‘Pre-Cambrian Sediments in the Adirondacks,” Proc. Am. Assoc. 
Adv. Sci., XLIX (1900), 167. 
2 Tbhid., 174. 3 [bid., 168. 
