;^0 NEW YORK STATE MUSEUM 



tabular and are arranged with their flat sides parallel in a rude flow- 

 ing arrangement. Under the microscope the labradorite presents 

 broad crystals consisting of multiple twins. As is so often the case 

 with rocks of the gabbro family, the labradorite is often charged 

 with minute inclusions, such as small brown rods, blebs and un- 

 identifiable dust. There seems no reason to doubt that as elsewhere 

 these minute particles are fragments of pyroxene, spinels and 

 ilmenite. The greatest difficulty in the accurate study of the anor- 

 thosite lies in the widespread granulation to which it has been sub- 

 jected. Exposures of perfectly crystallized rock are less frequently 

 seen. They are best developed in the southwestern and southern 

 portions of the quadrangle. Generally, however, the labradorite is 

 crushed and granulated around a central nucleus which may survive. 

 Stages can be traced from uncrushed originals, at times of very 

 coarse texture, with components comparable to rather coarse pegma- 

 tites ; through those whose feldspars have granulated edges ; through 

 others in whose granulated mass only remnants of the original 

 labradorite survive; to a final stage of complete granulation which 

 has left a pulp of small fragments of labradorite. 



Coincident with the crushing the circulation of groundwaters 

 seems to have taken place. In some specimens serious changes to 

 scapolite and calcite have developed, producing the old-time aggre- 

 gate called saussurile. The severe scraping, however, which the 

 region has suiTered from the continental ice sheet has tended to 

 clean away the softer varieties, and leave only bright, fresh rock. 



The coarser varieties of the less granulated or ungranulated anor- 

 thosites sometimes display the irridescent play of colors character- 

 istic of labradorite, a feature that may be observed in the smooth 

 bottoms of cascading brooks. Doubtless the name Opalescent river 

 for the stream on the west slope of Mount Marcy was suggested 

 by this feature. 



The anorthosite displays at times appreciable amounts of the 

 bronze-colored, faintly irridescent hypersthene, which attracted the 

 special attention of early observers. The hypersthene has irregular 

 outlines and under the microscope reveals the usual features of the 

 mineral. The chemical composition of a specimen from the summit 

 of Mount Marcy was determined in 1875 or 1876 by the late Prof. 

 Albert B. Leeds of Stevens Institute and is given in the valuable 

 paper entitled " Notes upon the Lithology of the Adirondacks," ^ to 

 which reference has already been made on an earlier page. 



1 Thirtieth Annual Report of the New York State Museum, 1876, p. 79. 

 The analysis of hypersthene is on page 25 of the repaged reprint. 



