Cushwg — Potsdam Boundary. 27 



bridge over the river on the town line. It seems further south than the one 

 mentioned by Winchell and from which his party obtained fossils.* The horizon 

 can not be far above the base of the formation, if undisturbed. 



Dikes. 



In following the boundary across Franklin county, eleven dikes were noted 

 cutting the Pre-cambrian rocks. These were all in the eastern half of the 

 county. In St. Lawrence not a single one was seen in the belt examined, and 

 Professor Smyth's work shows them to be rare in most of that county. In the 

 eastern Adirondacks they are exceedingly abundant and the same is true 

 further westward at the Thousand Islands as described by Smyth, who has 

 demonstrated them to be there of Pre-cambrian age.f 



Of the eleven dikes, ten were of diabase and one of syenite porphyry. 

 Nearly all the diabases contain olivine. One of them is noteworthy in that it 

 contains frequent large phenocrysts of a light green, almost non-pleochroic 

 orthorhombic pyroxene, probably enstatite, a mineral not of common occurence 

 in diabase, 



A more detailed description of the dikes will be printed elsewhere. 



* Minn. Geol. Surv., 21st Ann. Rep., p. 109. 

 t Trans. N. Y. Acad. Sci., Vol. XV, p, vL 



