Palache.] 



Liter zolite- Serpentine. 



■73 



blende in bladed crystals, and grains of pyroxene. Although in 

 the field this rock appears to be fairly uniform, when studied with 

 the microscope it is seen to vary considerably, exhibiting a grada- 

 tion of forms best described under two types. The first type, which 

 is the most abundant one, appears in thin sections as a uniform 

 granular rock composed of feldspar and hornblende in nearly equal 

 proportions with accessory magnetite. 



The feldspar, when fresh, exhibits the multiple twinning of 

 plagioclase. Several observations made on prismatic individuals, 

 which gave symmetrical extinctions on either' side of the trace of 

 the twinning plane, gave a maximum extinction angle of \J°, which 

 would place the feldspar near the acidic end of the labradorite 

 series. The feldspar contains clouds of dusty inclusions scattered 

 irregularly throughout its substance. Much of the feldspar is com- 

 pletely decomposed, chiefly to a non-polarizing, kaolin-like mass, 

 more rarely to a brightly polarizing granular aggregate, the nature 

 of which was not determined. 



The hornblende is a compact variety of that mineral, green or 

 greenish yellow in color, well marked by pleochroism, cleavage, 

 and extinction. It is always allotriomorphic to the feldspar. Pleo- 

 chroism is strong, the a ray very pale yellowish green, the b and c 

 rays dark green to yellowish brown, absorption a b t. The 

 extinction angle c: c was observed as large as 17 . Basal sections 

 exhibit characteristic prismatic cleavage of hornblende, and the same 

 cleavage is prominent in longitudinal sections. Irregular scattered 

 grains of magnetite form abundant inclusions in the hornblende. 

 As a rule the hornblende is free from alteration. The only evidence 

 of it observed is a fibration which in some cases affects the whole 

 crystal, accompanied by a change of color to a lighter green, and 

 by a less intense pleochroism. 



Magnetite, besides forming inclusions in other constituents of 

 the rock, occurs in large irregular grains having the ordinary prop- 

 erties of that mineral. 



In one slide a narrow vein was observed cutting the rock, which 

 was filled with a colorless, transparent mineral in radiating tufts of 

 acicular crystals. It possesses no cleavage, gives brilliant polariza- 

 tion colors, and extinguishes parallel to the prismatic axis. Double 



