54 NEW YORK STATE MUSEUM 



we can not understand why these very soluble minerals did not go 

 more largely into solution. Only by assuming some rather basic 

 plagioclase and then assigning arbitrarily the soda to nephelite or 

 analcite can a recasting be carried out and then the results in soluble 

 and insoluble must be ignored. Recasting has therefore not been 

 attempted. 



Bostonite. In the locality in the Johns Brook valley below the 

 junction with Ore Bed brook a loose piece or boulder was discovered 

 which was partly the dark rock regarded as an included mass of 

 Grenville gneiss, and partly a trachytic or bostonite dike. The dike 

 is of dense, felsitic texture, of pale green color, and about 35 mm, 

 or 1.4 inches, wide. A thin section of the two at their contact 

 revealed a remarkable plumose arrangement of the orthoclase rods, 

 as illustrated in plate 20 B. The border is 0.5 mm wide and is 

 succeeded by the normal bostonite which is an interlacing mass of 

 rods of orthoclase or perhaps anorthoclase. No dark silicates can be 

 detected. The rods have an extinction closely if not invariably 

 parallel with the elongation. The boulder is believed to have been 

 derived from the ledges in the immediate vicinity. The bostonite 

 dike is the first one met in the mountains south of the northern 

 border. Dikes of this character are most frequently observed cut- 

 ting the Paleozoic strata of the Champlain valley. Under the name 

 of syenite-porphyry, however, dikes which cut the old crystalline 

 rocks in Clinton county have been described both by A. S. Eakle ^ 

 and H. P. Gushing.' 



5 • 

 FAULTS, AREAL DISTRIBUTION OF FORMATIONS 



Faults. In an area of predominant massive rocks such as the 

 Mount Marcy quadrangle structure can not be worked out to any 

 sudh degree as is possible in sedimentary and contrasted strata. 

 Faults are the large structural features and yet they must often be a 

 matter of inference from the precipitous topography. Sometimes, 

 however, the crushed and decomposed rock can be seen. In the 

 summer of 1910 a borrow pit for improving the highway on the 

 west bank of the East branch of the Ausable river and just north 

 of the edge of the quadrangle, revealed a zone of thoroughly crushed 

 and kaolinized anorthosite, striking nearly parallel with the trend 



1 Eakle, A. S., Amer. Geol., July 1893, p. 34. 



- Gushing, H. P., Bui. Geol. Soc. Am., 9 1239-56, 1898. 



