390 QUICKSILVER DEPOSITS OF THF: PACIFIC SLOPE. 



traces of prismatic faces. A few isolated and well developed crystals have 

 been found sliowing-, in addition to the prismatic faces, terminal rhonibo- 

 hedrons. Cinnabar is often met with in dust-like aggregates disseminated 

 tlirougli quaitz, and with high powers this dust often resolves itself into 

 beautiful arborescent and capillary forms. In some hand specimens, too, 

 quartz may be seen reddened throughout the mass by disseminated ore. 

 These fibrous forms of the mineral are usually associated with concretionary 

 structure and the ore is often deposited in concentric layers parallel to those 

 of the accompanj'ing quartz or at the centers of geodes. Good crj'stals vis- 

 ible with the naked eye are very uncommon. Some such occurred in the 

 upper portions of the New Idria mine and were secured for the collection 

 by the courtesy of Mr. J. W. C. Maxwell. These are tabular and present 

 some remarkable characteristics, which will be discussed in a separate paper. 



Vast quantities of silica occur with cinnabar at most of the California 

 mines, and a large part of this material consists of mixtures of opal and 

 crystalline silica Such mixtures have long been known in geological liter- 

 ature as chalcedony, and the term is used in that sense in this volimie. 

 Professor Rosenbusch has shown, however, that the fibrous silica crystals 

 so common in these mixtures possess negative double refraction and are 

 distinct from quartz. He calls this mineral simply chalcedony.^ It seems 

 to me that the adoption of this name for this purpose will lead to much 

 confusion, since chalcedony has been freely employed in literature in the 

 other sense. A very slight modification of the term, however, would 

 obviate this olijection and would seem to escape an}^ fresh ones. Chalced- 

 on'ite at once suggests a mineral characteristic of chalcedon}", yet not iden- 

 tical with it. So far as I know it has not hitherto been employed in any 

 sense, and I venture to propose it for the anhydrous silica with negative 

 double refraction described by Professor Rosenbusch. 



In a great majority of cases the minerals immediately accompanying 

 the cinnabar are quartz and chalcedonite, and no case has been observed 

 in which the cinnabar particles were directly embedded in opal, although 

 the greater part of the area of some slides is occupied b}' the last-named 

 mineral. Minute cracks in the opal are often filled with crystals of cinna- 



I Mik. Pbys., vol. 1, 1885, p. 345. 



