DETAILED DESCRIPTION OF SLIDES. 107 



lites of green hornblende; it is strongly dichroitic, and extinguishes light 

 l^arallel to the fibers. There is also much epidote present in compara- 

 tively large crystalline masses. The dichroism, high colors of polariza- 

 tion, and the angles of extinction referred to the cleavages, leave no doubt 

 as to the mineral species. A few quartz grains are scattered through the 

 mass. The slide contains many minute scales of brown mica, but no well- 

 developed crystals. Its quantity is insignificant as compared with that of 

 the hornblende. 



The iron ore is very characteristic ilmenite, occurring in groups of par- 

 ticles which look as if they had been produced by chopping a larger mass, 

 and is accompanied by a little leucoxene. 



Slide 429. 3,000 feet southeast of Basalt Hill. 



Granitoid variety. — This is a piukish-gTay rock of granitoid structure, with 

 many lath-like feldspars, and a somewhat waxy look. In fact, its general 

 appearance resembles that of many diabases, but close inspection with the 

 unaided eye discloses small crystals of hornblende and mica. Under the 

 microscope quartz grains and some subsidiary minerals are added to the list. 



The hornblende is for the most part green and fibrous, a few patches 

 showing a tendency to brown shades. It is all partially decomposed, and 

 is far inferior to the feldspar in quantity, and has evidently crystallized later. 

 Only a few flakes of mica are visible. The feldspar is for the most part 

 polysynthetic, and the lamellae are excessively thin. The angles of extinc- 

 tion of the sections cut at right angles to the twinning plane indicate oligo- 

 clase as the species. There are no microlites of feldspar so developed as to 

 justify inferences concerning the species. A large part of the interstices 

 between the crystals are filled with quartz grains, which are evidently not 

 of secondary origin. They contain exceedingly minute fluid inclusions. 



There is a large amount of titanic iron in this slide, recognizable by 

 its cleavages and accompanying leucoxene. This latter mineral is intimately 

 associated with sphene, and indeed possibly passes over into it. Sphene 

 also occurs in patches independently of decomposed ilmenite. Though 

 determinable crystals are not visible, the characteristically irregular shape 

 of the masses both as to outline and surface, the high refraction, the feebly 



