404 Mr. J. A. Phillips on the Connexion of certain 



stantly accumulating on the sides, whilst a longitudinal central 

 crevice affords a passage for the escape of steam and boiling 

 water. This silica forms a series of semi-crystalline bands parallel 

 with the walls of the fissure, and often presents the cornby ap- 

 pearance peculiar to the beds of deposition so frequently observed 

 in mineral veins. 



At some distance to the west of the line of fissures now in a 

 state of activity, is another group of longitudinal siliceous de- 

 posits, also presenting a central crevice, from which steam and 

 carbonic acid still escape, although no longer traversed by hot 

 water. The silica from this locality contains oxides of iron and 

 manganese, together with traces of copper, and minute crystals 

 of iron pyrites. It has also been stated to be sometimes slightly 

 auriferous ; but I was unable to detect this metal in any of the 

 specimens selected for examination. 



The granite in which the fissures occur is covered, both on the 

 eastern and western sides of the valley, by a compact basalt. 



On analysis the deposited silica was found to contain from 

 4*72 to 6 per cent, of water ; and on being boiled, in a finely di- 

 vided state, with a strong solution of caustic potassa, 6*68 per 

 cent, of its weight was dissolved. 



Thin sections of this substance examined under the micro- 

 scope show the reniform structure of chalcedony, with patches of 

 amorphous silica, stains of oxide of iron, and geodes of crystal- 

 line quartz. Some few specimens also contain minute crystals 

 of pyrites. An attempt to give an idea of the appearance of a 

 section of the deposit from Steamboat Springs, when magnified 

 75 diameters, is made in Plate I. fig. 1. The crystals of quartz 

 lining cavities filled with amorphous silica become very distinct 

 when examined by polarized light ; but the chalcedonious mark- 

 ings then almost entirely disappear. 



At the " Sulphur Bank" on the shores of Clear Lake, Cali- 

 fornia, is a solfatara some six or seven acres in extent, where 

 a much decomposed volcanic rock is traversed by innumerable 

 fissures, from which steam, together with carbonic and boracic 

 acids, is continually issuing. Sulphur is deposited on the 

 sides of the crevices ; and gelatinous silica is found coating 

 chalcedony and opalescent silica in various stages of formation, 

 from the gelatinous state to that of the hardest opal. This 

 indurated silica is sometimes nearly colourless, but is more 

 frequently permeated by cinnabar and iron pyrites, or blackened 

 by a tarry hydrocarbon. Cinnabar is also found in striae, and 

 occasionally in veins, as well as in concretionary masses of consi- 

 derable size. 



Sections of chalcedony and semi-opal from this place, when 

 examined under the microscope, are often found to enclose crys- 



