234 WATSON AND CLINE ROCKS OF THE BLUE RIDGE REGION 



elusive, therefore, that the syenite of this region at least is Precambrian 

 in age. The same evidence applies to the granites, which in this region, 

 as in the northern Blue Eidge, are Precambrian. 



Where observed by us, the syenites of the Blue Eidge indicate, from 

 their relations to other rocks, Precambrian age ; but whether more than a 

 single period of intrusion is represented can not be answered from our 

 present knowledge of the field relations. It has been shown that the 

 syenite has not been equally metamorphosed in all places, but that the 

 rock varies greatly in structure from massive to highly gneissic, and some- 

 times almost schistose. The principle of inequality of metamorphism 

 alone, as expressed in the rock structures of this region, can not be used 

 as a positive criterion for regarding the rocks to be of more than one 

 period of intrusion, and therefore of different age, since in some areas 

 there can be traced syenite of the same mass whose structure ranges from 

 massive or only faintly gneissoid to highly gneissic, which is beyond 

 question the product of a single intrusion. On the other hand, there are 

 areas where such evidence is apparently lacking, and there may have been 

 more than a single period of intrusion of the syenite. 



It has been pointed out that the syenite shows variations in some local- 

 ities into more acid or granitic phases on the one hand and possibly into 

 more basic or gabbroic phases on the other, though the evidence is much 

 less conclusive for the latter. Not until the region has been covered by 

 thorough detailed study can 'the field relations of the several rock types 

 be finally settled, and whether there has been more than one period of in- 

 trusion of the syenite magma. It can be stated in conclusion, however, 

 that the relations show conclusively that the syenite of at least some of 

 the areas, and probably of all, is of Precambrian age. 



