FIFTH REPORT OF THE DIRECTOR I908 



9 



The Precambric rocks of the district comprise old sediments 

 of Grenville age and various igneous rocks of later age which have 

 freely cut the Grenville schists. The latter show a thickness of 

 several thousand feet, the greater part comprised in a series of 

 rocks of varied nature within which are thin bands of white lime- 

 stone and of quartzite, with much impure limestone. Unless the 

 series has been overturned the limestone is at the bottom and the 

 heavier quartzite at the top. The oldest of the igneous rocks is 

 the Laurentian granite gneiss, two considerable bathylithic sheets 

 of which lie in part in the mapped area and have been designated 

 the Antwerp and the Alexandria bathyliths. The gneiss is full 

 of inclusions of the Grenville rocks and clearly shows that it has 

 absorbed large quantities of the Grenville, with the production of 

 certain mixed rocks. 



The Grenville limestone has had a singular bleaching effect on 

 the red granite gneiss, turning it white. All the granite dikes that 

 cut the limestone are white and the edges of the larger masses have 

 the same color in the vicinity of the limestone; in fact the color 

 change in the granite is an infallible indication of approach to the 

 limestone, as repeatedly tested and proved in the field. 



An analogous change is observable at granite-quartzite contacts, 

 the granite becoming more acid, tending to bleach and showing 

 much more frequent joints. Over the area mapped the granite 

 gneisses seem to have been deficient in mineralizing fluids and have 

 not notably altered the Grenville rocks, the granite showing more 

 noticeable contact effects than the sediments. 



The later igneous rocks comprise a small streak of syenite, one 

 of diorite, a very small one of gabbro and a fairly large mass of 

 coarse red granite, the Picton granite, which seems to be the 

 latest of all, has a considerable extent of outcrop on Grindstone 

 and YVellesley islands and a much wider extent across the border 

 in Canada, though on the New York side it makes little show ex- 

 cept for an abundance of dikes which cut all the other rocks. On 

 the islands the granite is full of inclusions of the other rocks and 

 on YVellesley they are remarkably abundant, have retained their 

 original orientation, their strikes and dips being uniform and con- 

 cordant with those of the main areas of the rocks away from the 

 granite, so that they can be mapped into belts of quartzite, amphibo- 

 lites and granite gneiss with as much exactitude as though the later 

 granite were not present. This is taken to imply that here the very 

 roof of this portion of the bathylith is now at the surface, the in- 



