PETROLOGY 



319 



rieties of feldspar are 

 present; the most abun- 

 dant one is a polysyn- 

 thetically twinned calcic 

 plagioclase which occurs 

 in small grains; the 

 other is not twinned ex- 

 cept occasionally on the 

 Carlsbad law, and with 

 its low index of refrac- 

 tion has been identified 

 as orthoclase. The very 

 much clouded condition 

 of the ground-mass from 

 alteration products fre- 

 quently renders the ab- 

 solute optical indentifi- 

 cation of the feldspars 

 impossible. 



Augite, which is the 

 most abundant mineral 

 in the rock, appears to 

 be identical with that 

 occurring in the camp- 

 tonite, which is de- 

 scribed in detail below. 

 Hornblende was not 

 identified in any of the 

 thin-sections. Brown 

 biotite is an abundant 

 constituent, strongly 

 pleochroic (x and y = 

 dark brown, z = yel- 

 low), and occurs in hex- 

 agonal .plates and irreg- 

 ular shreds. Apatite 



OCCUrs in small pris- u/) Phenocrysl showing relations of analcite I in) to 



, - ill calcite (Ga) and ground-mans minerals (B). (Magnifies 



malic crystals included rion, 32 diameters.) Note iiu- well-developed crystal oul 



• ,, . . , . lines of the analcite. The calcite is secondary, probably 



i" UK' principal nun- along wilh ze ollte (Z) replacing analcite, which as an 



.,,.,1, | i -i , original constituenl occupied the entire area, 



erais, dux does not ap- </ ;l shows analcite occupying Interstices among the 



near fn hn oKnnrlnnf minerals of the ground-miss in the teschenite. (Magnl 



CO 1)0 anundant. ftcatton 00 diameters.) The analcite lias been partially 



nnmornna Un-hf replaced by calcite. Both (a) and (6) are typical of the 



numerous ngni- occur ,. ence oi an alcite In teschenite and camptonite. 



: re 2. Microscopic Sketches <>( Thin-sections <>f 

 Teschenite illustrating occurrence of inalcite 



The 



