AUGUST 19, 1921 WATSON: LAMPROPHYRE DIKE 343 



short prismatic nearly equant crystals up to 4 mm. in diameter. The 

 density of the specimen analyzed is 3.065. 



Microscopic characters. — In thin section the rock shows considerable 

 alteration, especially the goundmass, which is composed chiefly of 

 clay substance admixed with more or less chloritic and serpentinous 

 material, derived probably from either feldspars or feldspathoids or 

 both. Neither feldspar nor glass was observed in the thin section 

 studied, although both may have been present originally. The 

 groundmass of the composition indicated above was roughly estimated 

 at 30 per cent. 



Phenocrysts of biotite, augite, and altered olivine make up probably 

 60 per cent or more of the rock. Augite and much of the biotite 

 are usually fresh, but olivine is completely altered. The ore minerals, 

 titaniferous magnetite and apatite, the former predominating, make 

 up chiefly the remaining 10 per cent of the rock. No evidence of the 

 presence of sulfide minerals was indicated in either the hand specimen 

 or the thin section. 



Biotite is of the type usually characteristic of the lamprophyres. 

 It exhibits intense pleochroism in shades of light and dark brown, 

 and in part dark-colored resorption rims. Much of it is entirely 

 fresh, but some has altered to a faintly pleochroic red substance 

 without trace of cleavage. It usually contains some inclusions. 



Pyroxene is the variety augite near diopside in composition. Most 

 of it is colorless but some individuals show narrow faintly pleochroic 

 borders indicating some admixture of the aegirite molecule. Faint 

 violet shades indicating the presence of titanium are also noted. The 

 average extinction angle varies from 35 to 40 degrees. The augite 

 occurs in several generations and probably formed during the entire 

 period of crystallization of the rock. It is developed in subhedral 

 short nearly equant and in elongated prismatic (bladed) crystals, the 

 latter being predominant and so closely resembling in outline lath- 

 shaped plagioclase in diabase as to suggest diabasic texture. In 

 extreme cases the augite prisms are more than six times longer than 

 thick. A pronounced zonal structure is usually developed in the 

 nearly equant crystals, and some show hour-glass structure. Some 

 crystals show a green core that is faintly pleochroic and sharply 

 defined from the outer colorless part of the individual; others have 

 a pale green faintly pleochroic outer zone enveloping a colorless core. 

 The microscope indicates augite low in iron and in part titaniferous. 

 Cleavage is usually well developed in the nearly equant crystals and 



