84 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



usually have frequent offsets, and the crystallographic orientation, though 

 nearly the same for the whole group, is not absolutely uniform. It is ex- 

 tremely difficult to understand this building up of crystals from microlites; 

 yet perhaps in no other way could the formation of garnets, tourmalines, 

 and a long list of other minerals be exj)lained in strata which can never 

 have been reduced to a plastic state. 



The commonest feldspar species in the altered sandstones and the 

 granular rocks is oligoclase, but andesine is probably also common. Many 

 angles of extinction, referable to andesine, -have been observed, and in 

 one case a separation and chemical analysis showed the presence of this 

 feldspar. Labradorite is found in the gabbroitic rocks and in at least one 

 pyroxene rock where the bisilicate is not diallage. Orthoclase has been 

 proved chemically to exist in one glaucophane rock, and albite in a feld- 

 spar-augite-hornblende rock. There may be more albite than has been 

 detected, since it cannot be recognized with ease by optical means in the 

 presence of oligoclase. 



Rutiie In some of the schists and amphibolites numerous masses of 



a bright-brown, anisotropic mineral were observed. Manj^ of these masses 

 are prismatically developed, though the edges and corners are somewhat 

 rounded. They are monocln-oitic and extinguish light when parallel to the 

 principal sections of the nicols. The interference colors are scarcely dis- 

 liiiguishable from those observed in ordinary light. A small amount of this 

 mineral separated from an amphibolite proved on chemical examination to 

 be titanic acid. The absence of dichroism and of brilliant colors of inter- 

 ference shows that it is nut brookite. The prismatic develo2)ment excludes 

 anatase,^ while its characteristics correspond exactly to those of rutile. Some 

 of the masses of rutile are partially decomposed to a light-colored, clouded 

 substance similar to leucoxene. 



iimenite. — Titauic Irou is very abundant in some of the groups of gran- 

 ular, metamorphic rocks, and the associations are such as to lead to the sup- 

 position that it has been formed at the same time with the bisilicates. The 

 characteristic triangular grating of the iimenite is much more common in 

 these rocks than it usually is in eruptive masses. Tlie iimenite is fretpiently 



' See a report of an iuvestigatiou by Thiii-ach: Neiies Jahrbuch iiir MiDeial., vol. 2, 1885, p. ;198. 



