XVII PLUTONIC ROCKS 



251 



position products. The lamellar extinctions indicate andesine- 

 labradorite. The quartz occurs mostly in nests. The pyroxene 

 is formed of large grains of both the monoclinic and rhombic 

 types. 



The Diorites. — The rock forming blocks in the Vunimbua 

 River has a specific gravity of 278 to 2-8. It is a pretty rock 

 show^ing long black blades of hornblende, 10 mm. in length, in an 

 opaque felspar base. In the slide the hornblende, which is dark 

 brown and markedly pleochroic, shows six-sided sections with 

 characteristic prismatic cleavage lines, the longitudinal sections 

 giving extinctions up to 15°. The borders in some cases display 

 traces of resorption. The felspar (plagioclase) is in the form 

 usually of broad regular crystals, 3 to 4 mm. in size, and giving 

 extinctions of andesine-labradorite (28°) ; they are " clouded " 

 through the presence of fine alteration products associated with 

 numerous fissures. The relation between the hornblende and 

 the plagioclase is not constant. This appears to be partially due 

 to the occurrence of traces of a groundmass. 



The diorite forming blocks in the agglomerate of the coast 

 cliffs, west of the Salt Lake Passage, is a remarkable rock showing 

 large blackish hornblende crystals, in the shape of blades 25 mm. 

 long and 3 or 4 mm. broad, set in a base of opaque plagioclase 

 felspar which surrounds the hornblende. The last-named is deep- 

 brown, very pleochroic, yields extinctions up to 22°, and displays 

 but little evidence of resorption. The plagioclase is irregular 

 in shape and exhibits broad lamellae giving extinctions of acid 

 labradorite (28° — 30°). It is traversed by numerous fine fissures 

 filled with decomposition products and contains abundant dust-like 

 materials. (Spec. grav. 2-8). 



