GEOLOGY OF THE LAKE PLACID QUADRANGLE 2$ 



ledges on top of the mountain spur i^ miles northeast of the 

 summit of Little Whiteface mountain. 



Microscopic features. The thirteen thin sections listed in table i 

 are from specimens chosen to illustrate the usual range in mineral 

 composition of the Whiteface anorthosite of the quadrangle. 

 Altogether some sixteen or seventeen mineral species were noted. 

 Plagioclase feldspar, chiefly labradorite, always makes up the mam 

 bulk of the rock, being seldom less than 85 per cent. Oligoclase 

 and andesine commonly occur in small amounts. So far as 

 observed, the feldspar never shows the black dustlike inclusions 

 so common in the labradorite of the Marcy anorthosite. Some of 

 the large phenocrysts probably would show them, but none of these 

 appear in the thin sections examined. 



Greenish gray, or less commonly a nearly colorless, monoclinic 

 pyroxene (in one case some diallage) appears in all the sections 

 except no. 6, in which slide the chlorite was evidently largely 

 derived from pyroxene. In no. i8 all but i per cent of the pyroxene 

 IS nearly colorless and the large amount is very exceptional. 

 Pyroxene is the second most abundant constituent of the rock. 



Next in amount comes the hornblende, never more than 5 or 6 

 per cent. Its pleochroism is usually greenish yellow to deep green 

 or brownish green, and the cleavages are good. 



Biotite was noted in only one slide, but it was occasionally noted 

 in the field. Red garnet occurs in several slides, but it is present 

 as scattering grains through the Whiteface anorthosite in many 

 localities. Ilmenite (or magnetite) is generally present in tiny 

 grains. Tiny prisms or rounded grains of apatite, zircon or titanite 

 often occur in very slight amounts. Hematite, muscovite and quartz 

 (probably secondary) are rare. 



So far as can be judged by a study of hand specimens and thin 

 sections, it seems to be quite the rule that the Whiteface anorthosite 

 is less granulated than the Marcy anorthosite. Only exceptionally 

 does the Whiteface type appear to have been severely crushed. A 

 possible explanation will be offered beyond under the caption 

 " Foliation." 



Chemical composition of the anorthosite. Analyses of the 

 Marcy type of anorthosite from the summit of Mt Marcy, and of 

 the Whiteface type from the summit of Mt Whiteface, have been 

 made for, and described by, Professor Kemp.^ They are as follows : 



1 N. Y. State Mus. Bui. 138, p. 32-34 and 36-37. 1910. 



