96 QUICKSILVER DEPOSITS OF THE TAOIFIO SLOPE. 



nature is uncertain. Distinctly developed nacrite has been detected in only 

 one of the rocks. 



Much of the quartz in the pseudodiabases is secondary, but in a few 

 cases quartz grains appear to have formed contemporaneously with the 

 feldspars. They carry a few fluid inclusions. 



No rhombic pyroxene lias been detected. In the extremely rare cases 

 in which crystals of pyroxene extinguish light when parallel to the princi- 

 pal nicol sections, there appears no difference between their dichroism or 

 other properties and those of the other pyroxenes in the same slide. The 

 proportion of cases in which the extinction of the p)'roxene cr^-stals sensi- 

 bly coincides with the principal axis is very small and not greater than 

 might be expected in the case of a monoclinic mineral. Both angite and 

 diallage occur, but no sharp line can be drawn between them. Linearly 

 all cases the clinopinacoidal cleavage of the larger crystals seems well 

 developed. P}'roxene evidently develops early and vigorously in the rocks 

 undergoing recrj'stallization and tends to the formation of porphyritic 

 crystals. Well -developed crystals are not very conmion. 



Aniphibole occurs as brownish-green crystals, formed contenqjorane- 

 ously with the angite and more abundantl}' as unmistakable uralite. A 

 few of the pseudodiabases also carry glaucophane. 



Examples. — No. 21, Coast Rangcs, from near Mt. St. Helena, is a gray, 

 rather fine-grained rock, which, on close inspection, appears crystalline. 

 Under the microscope it is seen to contain much augite and hornblende, 

 together with feldspar, both polysynthetic and nionosynthetic, and an unu- 

 sually large quantity of zoisite. It is also unusually free from decomposi- 

 tion products. This rock represents both the pseudodiabase and pseudo- 

 diorite, betvreeu which there is no sharp division. 



The hornblende is in part of a clear, light, but not vivid, brown color, 

 with very moderate dichroism. Tliis variety occurs in well developed crys- 

 tals, giving normal extinctions and cross-sections. Curiously associated 

 with it is a light- green variety. Many of the crystals are in pai't brown 

 and in part green, the division being a sharp, straiglit line. The different 

 portions of such composite crystals extinguish simultaneously, but usuall}- 

 give very different interference colors. The cleavages are continuous from 



