244 J. D. TRUEMAN 
matter what the depth, to have been under considerable pressure. 
Weinschenk has more than any other emphasized the importance 
of pressure during the intrusion of primary gneisses and suggests 
as proof the wide schistose zones about such bodies. As showing 
that these were formed during the intrusion, he states™ that the 
schistosity of the contact rocks is generally parallel to the border 
of the batholith and that tourmaline needles, which he considers 
were formed by pneumatolitic action, frequently le across the 
the plane of foliation and, accordingly, were formed later than the 
schistosity. 
It seems not illogical to assume that the movements which were, 
apparently, present late in the period of consolidation should 
have sometimes continued after portions or the whole of the rock 
had completely solidified. If such were the case there would 
result considerable recrystallization and granulation so that typical 
crystalloblastic or cataclastic textures might be superimposed on 
that resulting from primary consolidation. It follows that the 
mere presence of a metamorphic texture is no proof that the band- 
ing of a gneiss is not of primary origin. When, however, such rocks 
are characterized by true igneous texture, as would be the case if 
the movements ceased early in the period of consolidation, there 
seems no reason why this feature should not be regarded as proof 
that the banding was of primary origin. The use of texture as a 
criterion for the identification of primary gneisses seems on the 
whole, then, to be of only limited application. 
THE USE OF ZIRCON AS A CRITERION FOR THE IDENTIFICA- 
TION OF THE ORIGIN OF FOLIATED ROCKS 
INTRODUCTION 
It was noted in the general introduction that the criteria which 
have been proposed for determining the igneous or sedimentary 
origin of metamorphic rocks can seldom be employed decisively. 
This is largely because the evidence used is generally indirect, and 
its application as proof of original character is frequently dependent 
upon inferences which have as yet not been shown to be correct. 
™ Congrés géol. inter., compte rendu, session VITI, I (1900), 330, 337- 
