MICROSCOPICAL CHARACTER OF ORE. 391 



bar and of silica, and these indicate that the deposition of opal preceded 

 that of such cinnabar and the accompanying quartz and chalcedonite. 

 The mixture, however, is sufficiently intimate to compel the conclusion that 

 opal deposition cannot have been an independent process, but only an early 

 stage of the same process by which cinnabar was ultimately deposited. It 

 is possible that there may be cases in which cinnabar is directly embedded 

 in opal, though this association is not represented among the slides, and it is 

 certain from field observations that very little of the cinnabar can occur in 

 this way. It seems more probable that the conditions attending the actnal 

 crystallization of the cinnabar were also favorable to the cr}-stallization of 

 the silica. Calcite and dolomite, especially the former, are found in a large 

 proportion of the slides. The deposition or formation of more or less dolo- 

 mitic carbonates appears from the slides to have preceded the deposition of 

 opal, quartz, chalcedonite, and cinnabar in most cases, but in some instances 

 the carbonates fill interstices in chalcedony, showing tliat carbonates were 

 deposited at distinct periods. Cinnabar is sometimes deposited in direct 

 contact with the carbonates, but occurs in this way much more rarely than 

 in quartz. Field observations also show an irregularity corresponding to 

 that observed under the microscope. In most of the mines silica predomi- 

 nates in some portions and carbonates in others. This variation is not only 

 characteristic of the deep mines, but also of Steamboat Springs, where 

 some areas of the spring deposits are almost pure chalcedony, while others 

 consist largely of carbonates. The intimate association of cinnabar with 

 opal and carbonates is of course sufficient to show that the ore is deposited 

 from solutions. 



Inclosing rocks.— The couutry rock of the cinnabar deposits Is of the most 

 varied character, and I am unable to see that, excepting from a mechanical 

 point of view, the rock has exerted any influence on deposition. The old- 

 est rock In which cinnabar occurs Is granite, in which the main part of the 

 deposit at Steamboat Springs is found. Ilie ore occurs in every variety of 

 the early Cretaceous rocks, in unaltered sandstones, and also in phthanite, 

 pseudodiabase, pseudodiorite, glaucophane schists, and serpentine. The 

 most important deposits occur in the metamorphosed rocks, but this seems 

 to be due only to their hardness, as will be explained a little later. Cluco 



