394 QUICKSILVER DEPOSITS OP THE PACIFIC SLOPE. 



Califoi'iiia serpentine. This structure is also seen to be gradually effaced 

 as the quantity of opal increases. The silica solutions .seem clearly to have 

 permeated more or less fractured serpentine, extracting the bases and de- 

 positing the opal, the course of decomposition being the same indicated for 

 the silicified serpentine of the Bijhmerwald by Schrauf ' and for the rocks 

 at Bilin by Doelter.- Remnants of hornblende have been detected in one 

 of the opals and are probably explained on the supposition that pseudo- 

 diorite is subject to replacement by opal. In one case, from the Lake mine, 

 glaucophane rocks have also undergone a similar change, glaucophane and 

 distinct traces of the structure of the schist in which that mineral occurs 

 remaining in a portion of the slide, but gradually passing over into micro- 

 cry.stalline quartz. There is similar evidence that chloritic sandstones have 

 been impregnated with opal, and one such specimen from Knoxville con- 

 tains perowskite. It forms small cubes of a violet-brown color, which 

 refract light strongly. Twins formed by two cubes united according to the 

 spinel law and one form similar to the ];)entagonal dodecahedron were also 

 observed. It has been found in the Coast Ranges only in tliis one specimen, 

 where it formed at the same time as the opal. 



HYPOTHESIS OF SUBSTITUTION.. 



Substitution of ore for rock doubtful. — Aluiost froui tlie beiiinninff of the investiara- 

 tions described in this volume my attention has been directed to cases of 

 replacement, including those by cinnabar; but I have entirely failed to find 

 any valid evidence that this process has gone on to any considerable extent. 

 On the contrary, careful stud}* of much ore in place and under the microscope 

 seems to show that the cinnabar and the gangue minerals immediately accom- 

 panj'ing it have been deposited exclusively in pre-existing openings. These 

 are usually fissures in the rocks, and most large bodies of ore seem to me to 

 consist of crushed rock the interstices of which have been filled with cinna- 

 bar, quartz, and carljonates. I have never met with an instance in which the 

 ore masses were bounded by rock the surffice of which presented the joeculiar 

 pits and corrugations so characteristic of corrosion. Where compact rock is 



' Tschermak's Miueral. Mittlieil., 1873, p. 13. 



-Zeitsch. fur Krys. uud Miu., Groth, vol. G, 1881, p. 321. 



