366 GEOLOGY OF THE EUEEKA DISTRICT. 



fluely fibrous, yellowish green mineral, faintly pleochroic and with a rather high index 

 of refraction, whose angle of extinction is over 20 in some sections, and which must 

 therefore be a fibrous amphibole. Its fibers start from transverse fissures and run 

 parallel to the vertical axis of the crystal for a short distance, thus forming a net 

 work, the meshes of which are filled with a colorless substance without noticeable 

 action on polarized light, who ;e granular texture and botryoidal form suggest amor- 

 phous quartz. Scattered through these minerals are numerous opaque and transpar- 

 ent globulites of indeterminable nature, together with ferrite and red oxide of iron 

 directly traceable to the black border, from which it projects into the crystal in thin 

 branching plates (35). A more intimate mixture of the two principal decomposition 

 products has taken place in thin section 41, most of which has fallen out in grinding. 

 In thin sections 42, 42, the hornblende sections are without black borders and the 

 alteration has advanced farther, resulting in a yellowish green, fibrous chlorite, with 

 extinction parallel to the fibers, which has spread through the groundmass of the 

 rock, imparting to it a green color. 



Biotite is less abundant than the hornblende, though in much larger crystals. 

 It occurs in six-sided crystals with very marked pleochroism and strong absorption, 

 being deep red when the section is parallel to the base, and in oblique sections in 

 ordinary light orange, yellow, and green (35). The interference figure shows a small 

 optical angle, which varies somewhat in different thin sections, the plane of the optic 

 axes being perpendicular to that of symmetry in the crystal. Twinning parallel to 

 <x>P (110), where the composition plane is the base OP, is frequent. It is recognized 

 by bands with different angles of extinction in sections slightly inclined to the base, 

 and by the different positions of the interference figures in such sections, and also by 

 difference in the pleochroism in some sections nearly perpendicular to the base. 

 There are numerous black microlites arranged in lines perpendicular to the six faces 

 of the mica crystal, besides irregularly scattered prisms of apatite, and more rarely 

 zircon. In thin section 39 there are portions of the groundmass, each containing one 

 or more apatite crystals. The biotite has remained perfectly fresh in most of the thin 

 sections, though the hornblende, has been entirely decomposed. In section 37 the 

 biotite, though bleached out and stained yellow by iron oxide, still retains its optical 

 properties. 



Quartz appears as a very inconstant accessory ingredient, being wholly wanting 

 in the form of primary phenocrysts in the typical crystalline hornblende-[mica]-aude- 

 site 41, 35, but occurring in abundance in the glassier variety from east of the Pinto 

 |{<i;nl (39), where it is in rounded grains and fragments, the largest 3 mm in diameter. 

 They carry numerous dihexahedral glass inclusions with gas bubbles, around which 

 in polarized light the quartz shows the phenomena produced by strain. Other small 

 grains and fragments are found sparingly in thin sections 42 and 38. Microscopic 



