THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 21 



larly free from feldspathic and ferromagnesian impurities. The 

 microscope shows it to be composed almost wholly of calcite (plate 

 I, upper figure), in rather uniform interlocking grains, with a small 

 quantity of minute scapolite crystals. Accessory graphite flakes are 

 segregated mostly within the scapolite, but are also found dissemi- 

 nated throughout the calcite. Coccolite is rarely present in mega- 

 scopic crystals. 



The Crary Mills occurrence is on the left bank of Grannis brook, 

 at the southern corner of Potsdam township. The dark silicates 

 occur abundantly in streaks and irregular blotches, and are generally 

 accompanied by a considerable quantity of feldspar. A thin section 

 (plate I, lower figure), taken from a typical silicate knot at this 

 locality, shows phlogopite and orthoclase predominating, with a 

 smaller quantity of calcite, microcline, muscovite and actinolite, the 

 accessories being a few graphite flakes, and a crystal or two of 

 titanite. The mode of occurrence of these dark-colored bands, 

 showing every gradation to the light-colored, more calcareous por- 

 tions, is such as to suggest that they are recrystallized impure 

 laminae which have been greatly broken and torn apart by move- 

 ments within the semiplastic limestone. There is nothing in the 

 occurrence which suggests that these minerals are due to the intro- 

 duction of iron and alumina by dififusion from adjacent granitic or 

 gabbroic magma. They seem on the other hand to be recrystallized 

 sedimentary matter, as much so as the limestone in which they are 

 found. 



Another convincing illustration of the interlamination of impure 

 or siliceous layers in the limestone is found in the northwest portion 

 of the southwest ninth of the sheet, along the western and southern 

 borders of the Church Brook limestone. A typical outcrop of this 

 vicinity is shown in plate 2, upper figure. The contorted fragments 

 are a siliceous biotite gneiss which, having weathered in relief and 

 assumed a dark color, stands out in striking contrast to the lighter 

 colored and more easily eroded limestone. This alternation of 

 gneissic and calcareous layers characterizes what is evidently a 

 transitional member between the purer limestone below and the 

 rusty gneisses above. 



Tremolite has a sporadic but widespread distribution ; in nearly 

 every limestone area ledges are found with numerous radiating clus- 

 ters of transparent or translucent blades of this mineral. A's a rule 

 it is considerably weathered and seldom offers more than a transient 

 temptation to the mineral collector. In Van Rensselaer Creek 



