﻿GEOLOGY OF THOUSAND ISLANDS REGION IO3 



border phase is considerably metamorphosed and converted into 

 a thorough gneiss, with the augen (the uncrushed remnants of 

 original large feldspar crystals) alined in the direction of the 

 foliation. This also is coincident with the direction of foliation 

 in the Grenville and Laurentian rocks. While it is true that the 

 metamorphism exhibited by the syenite is not as severe in degree 

 as that shown by the other two groups, it is clear that there was 

 severe compression of the region at, or after, the time of syenite 

 intrusion, and compression under quite similar conditions as 

 regards overlying load. 



The Theresa syenite does not appear so foliated as does the 

 Alexandria, chiefly because of difference in composition, which 

 shows itself mineralogically in the much slighter development of 

 hornblende and mica, the rock consisting largely of feldspar. It 

 also lacks the coarsely porphyritic phase. Foliation is therefore 

 much less prominent, though the rock shows crushing and recrys- 

 tallization in degree quite comparable with the other. It has 

 therefore likely experienced compression of substantially equiva- 

 lent amount and duration, but its composition prohibits good 

 foliation development. 



Picton granite. This, the latest of the early intrusives of 

 the district, shows little or no foliation, and to the eye gives 

 little evidence of crushing, as if the intrusion was wholly sub- 

 sequent to the great squeezing of the region. The thin sections 

 bear out this impression. 



This evidence would seem to indicate compressive stresses ap- 

 plied at intervals through a considerable length of time during 

 the region's very early history, with gradual cessation, and that 

 the foliation structure in the Grenville and Laurentian rocks 

 must be due to something more than the pressure and heat fur- 

 nished by the intrusion of the Laurentian granites. 



Joints 



The clean-cut divisional planes, usually highly inclined, which 

 occur in most rocks, are termed joints. While generally vertical, 

 or nearly so, they may have any inclination. In a " joint set " 

 the divisional planes show a close approach to parallelism, both 

 in trend and in inclination. In most regions more than one set 

 is present. When there are two, the usual condition is that they 

 are approximately at right angles to one another. Often there 

 are more than two sets as is the case in our region here. When 

 four sets are present it is usually found that they are separable 



