r34 NEW YORK STATE MUSEUM 



spar individuals, working thence inward, underground water 

 being the agent which effected the alteration, the action occurring 

 long ago and in depth. The dark silicates have gone to chlorite 

 and serpentine, the feldspar to saussurite, much zoisite having 

 developed along with chlorite^ muscovite or paragonite, and a 

 colorless, weakly refracting, strongly doubly refracting biaxial 

 mineral, with highly inclined extinction and rather small axial 

 angle, whose identity has not been determined. Considerable 

 epidote has developed locally. In the cut is some white quartzose 

 gneiss, most probably an inclusion from the adjacent Grenville 

 rocks, and some coarse granitic gneiss, likewise a probable 

 inclusion. There is also an 18 inch dike of gabbro norite, prob- 

 ably a thoroughly metamorphosed dike of the hyperite gabbro. 



The next three cuts are of small extent, but all show anorthosite 

 gabbro rather than anorthosite^ both fine and coarse varieties 

 <)ccurring. 



There is a 30 yard cut in coarser and more typical anorthosite 

 than the preceding 4J miles from Saranac. A diabase dike in 

 three branches is exposed in this cut, these dikes being rare so 

 far west. 



The last cut is mile from the Lake Clear depot (984). Here 

 are the same phases as before, though the granular rock differs 

 from that in the previous exposures, both to the eye and under 

 the microscope. To the eye the main difference is the absence of 

 garnet, corrosion rims not being present, so that the rock lacks 

 the peculiar appearance which these invariably give. Apatite, 

 magnetite, augite, hypersthene, hornblende and a basic plagioclase 

 are the mineral constituents, and the rock is quite certainly more 

 basic than any of the preceding. The feldspar makes from Sofo 

 to 90fo of the rock, is beautifully twinned with both albite and 

 pericline lamellae^ and shows maximum equal extinction in sec- 

 tions perpendicular to the albite twinning plane of 33°, with 

 nearly all such sections in the slide showing from 25° to 30°; 

 60 that the feldspar is in all probability bytownite, instead of the 

 usual acid labradorite of the anorthosites. 



Summary of section. The main rock in this section along a 

 part of the bo'undary is anorthosite gabbro, rather than anortho- 



