22 NEW YORK STATE MUSEUM 



feet wide where measured, bears N. 25° E. and seems quite vertical. 

 No very fresh material could be obtained from it and the alteration 

 which it has undergone has changed its original black color to a 

 dark green. Like all the wide dikes it is of sufficiently coarse grain 

 so that the different minerals may be made out by the eye. Very 

 fresh, glittering, lath-shaped feldspar crystals are abundant, along 

 with much coarser feldspar which is somewhat altered and dull. 

 No olivine can be seen, and the rock is the ordinary feldspar- 

 augite-magnetite mixture usual in these dikes. 



The dike does not trend with the rock strike of the adjacent 

 rocks which, at the locality, is north-south. Where seen it cuts 

 across Grenville quartzite and amphibolite and adjacent syenite. 



Structures of the Precambrian Rocks 



The Precambrian rocks are exposed over such a comparatively 

 small area that study of the structures shown leads to little that is 

 of value and nothing that is new. Their most prominent structure 

 is foliation. The Grenville rocks are believed to be greatly folded, 

 but little that is definite concerning the folds can be made out from 

 the study of such a small area. 



Foliation. In the Ogdensburg region the foliation cleavage 

 which has been- developed in the Grenville rocks under meta- 

 morphism is closely parallel to the bedding, and this seems to be the 

 case throughout the Adirondack region. Strike and dip readings 

 on the one therefore correspond to those on the other. 



The general strike of the Grenville rocks of the district is north- 

 east-southwest. But there is much local variation from this general 

 direction. Our readings vary from N. 10° W. to N. 85° E. Often 

 there is rapid local variation due to folding. Along the Oswe- 

 gatchie nearly north-south strikes prevail ; along the east margin of 

 the Ogdensburg sheet the strike has swerved to the northeast and 

 beyond, and so continues on to the Canton sheet ; passing westward 

 toward and on to the Brier Hill sheet, it has gone back to the north- 

 east. This is also its general direction in the Thousand Islands 

 region. 



The general dip is to the northwest. It is usually steep, in excess 

 of 45°, and locally becomes vertical. Locally also it varies, passing 

 from northwest to southeast and back again within narrow limits, 

 indicative of local plications. The only part of the area within 

 which southeast strikes prevail is a narrow belt on the east side of 

 the Oswegatchie, on the south margin of the Ogdensburg sheet. 



