﻿44 NEW YORK STATE MUSEUM 



illustration is that of the granite gneiss in the extreme southeast 

 corner of the Alexandria sheet, which forms a wonderful cliff 

 along the Indian river. 



In quite a number of localities syenitic rocks were found, always 

 of trifling extent, and with field relations wholly indeterminate. 



At the west end of the upper bridge at Theresa, is a small intru- 

 sion of gabbro, which is but little mashed, and has some features of 

 interest in that it recalls the anorthosites and gabbros of the general 

 Adirondack region, and is the only representative of these rocks 

 seen here. It is a dark colored rock, showing numerous, glittering, 

 lath-shaped feldspars up to an inch in length, on broken surfaces. 

 It is made up of feldspar (labradorite), augite, hypersthene and 

 hornblende, with considerable magnetite, and a little pyrite and 

 apatite as accessories. The feldspar constitutes from 60 to jofc of 

 the rock. In composition therefore it is distinctly a gabbro, 

 though with more abundant feldspar than the usual Adirondack 

 gabbro. Yet, in spite of the coarsely lath-shaped feldspars the 

 structure is more nearly that of a gabbro than a hyperite, recall- 

 ing in this respect the anorthosite-gabbros farther east. 



Diabase. Cutting all the other Precambrie rocks of the region, 

 occasional dikes of trap rock are found. The fact that they cut all 

 the other rocks shows that they are younger, but it can also be 

 shown that they are much younger than the other igneous rocks, 

 though nevertheless older than the Potsdam sandstone. They 

 are found only in the form of dikes, which are lava-filled fissures 

 that in general represent plugged channels of ascent of the 

 molten rock, leading downward to some source of supply of the 

 material, and tending upward toward the surface. The dikes have 

 chilled borders, showing that the inclosing rocks were compara- 

 tively cool and hence at no great depth beneath the surface at 

 the time of solidification. Furthermore they show no sign of 

 having undergone the kind of deformation which all the other 

 igneous rocks have experienced in greater or less degree, a kind 

 which takes place only at considerable depths. Since the dia- 

 bases cooled much nearer the surface than the granites and 

 syenites, a long time interval of surface erosion during which a 

 considerable rock thickness was worn away from the surface, 

 must separate the two. 



In the district mapped these dikes have a somewhat unequal 

 distribution. They are most abundant on Grindstone island, seven 

 having been noted there, mostly of large size, none of them less 

 than 20 feet wide, and ranging from that up to 100 feet in the case 



