THE VALUE OF CERTAIN CRITERIA FOR THE DETER- 
MINATION OF THE ORIGIN OF FOLIATED 
CRYSTALLINE ROCKS. I 
J. D. TRUEMAN 
University of Wisconsin 
OUTLINE 
INTRODUCTION 
Review of criteria which have been proposed for the determination of the 
origin of foliated rocks 
Scope of paper 
TEXTURE AS A CRITERION FOR PRIMARY GNEISSES! 
Outline of discussion 
The distinction between igneous and metamorphic textures 
The significance of the elongated habit assumed by minerals when growing 
under certain conditions 
Elongation of minerals under viscous conditions 
Elongation of minerals on account of differential pressure 
Conclusions regarding the causes of mineral elongation 
Inferences from mineral elongation with regard to the texture of 
primary gneisses 
Conclusions regarding the texture of primary gneisses from the mode of 
their intrusion 
USES OF ZIRCON AS A CRITERION FOR THE JD SERA ATNON OF THE ORIGIN 
OF FOLIATED Rocks 
Introduction 
Identification of zircon 
The manner of its formation and its capacity for resisting alteration 
The significance of the presence or absence of zircon in a rock 
The significance of the character of the zircon grains 
Conclusions regarding the use of zircon as a criterion 
Use of other minerals similar to zircon 
CHEMICAL COMPOSITION AS A CRITERION FOR THE DETERMINATION OF THE 
IGNEOUS .OR SEDIMENTARY ORIGIN OF FOLIATED ROCKS 
Views of other writers 
The alteration of quartzite to sericite schist at Waterloo, Wis. 
The character of the chemical changes during the development of foliation 
Can chemical composition be used as a criterion for the determination of 
the origin of foliated rocks ? 
GENERAL SUMMARY 
t By “‘primary gneiss” the writer understands a banded crystalline rock of igneous 
origin whose banding was produced prior to the complete solidification of the rock. 
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