r54 



NEW YORK STATE MUSEUM 



minute description. Unfortunately it has not been investigated 

 chemically. 



Mineral content 



The primary minerals (those formed by the original cooling of 

 the rock) which have been noted in the Rand hill gabbro are as 

 follows, in relative order of abundance: plagioclase feldspar 

 (usually labradorite), augite, quartz, hornblende, hypersthene, 

 apatite, ilmenite (or titaniferous magnetite), zircon, pyrite, pyr- 

 rhotite and titanite. Secondary minerals are garnet, hornblende, 

 biotite and quartz. There are a number of other minerals present 

 which have resulted from the excessive alteration or decomposi- 

 tion of the foregoing, but they are the usual ones and do not need 

 farther mention. The order of crystallization was the usual one, 

 first the zircon and apatite, followed by the iron ores, then the 

 hypersthene, augite and hornblende, whose relative order is not 

 apparent, next the feldspar and last of all the quartz. The periods 

 of formation of the dark silicates and of the feldspar largely over- 

 lap. 



The feldspar is in all respects like that of the anorthosites. It 

 is full of inclusions of uncertain nature, mostly of short, stout, 

 opaque rods. It is usually a well twinned plagioclase, albite 

 twinning being well nigh universal, that after the pericline law 

 often combined with it, but Carlsbad twinning being very excep- 

 tional. Sections cut perpendicularly to the twinning plane show 

 maximum extinctions of from 20° to 24° in all the slides. This 

 makes it reasonably certain that no feldspar more basic than lab- 

 radorite is present, though not precluding the possibility of a 

 more acid one being at hand in slight amount. Five cleavage 

 pieces (parallel to the M cleavage) from different augen give ex- 

 tinctions of from — 15° to — 17° from the edge of the P cleavage, 

 with an average of — 16°. which is precisely the angle of typical 

 labradorite, Ab T An,. 



Of the pyroxenes hypersthene is relatively rare, a light green, 

 non-pleochroie augite being the usual mineral. This is identical 

 with the usual augite of the anorthosites and has been often 

 described. 



