50 GEOLOGY — ONYX OPAL AND AGATE — SILICIFIED WOOD. 



burrstone; the spaces occupied by the pumice being empty, or containing a small flock of light- 

 reddish matter, the remainder having been dissolved and carried off, or chemically combined. 

 The cavities which the mineral in this state exhibit are angular, their walls having a radiated 

 and crystaline structure, apparently produced by an effort of the particles composing them to 

 assume a spherical form with a radiated arrangement. Not unfrequently a small, hollow, or 

 solid sphere is formed on all or several of the walls of the cavity. The third stage of meta- 

 morphosis exhibits these cavities filled with onyx or opal — more rarely with agate — the rock 

 having assumed a peculiar concretionary structure. The onyx consists of bands of red, white, 

 green, or translucent silica, forming specimens of great beauty. 



These layers would seem to have been deposited parallel with the horizon, as the bands of 

 color in the onyx, filling different cavities of the same mass, are accurately parallel. In some 

 cases the cavities are but partially filled, several bands stretching across from side to side, with 

 open spaces between them. 



The opalescent silica exhibited considerable variety, some being milk white and opaque, 

 apparently retaining a considerable portion of felspathic material which originally filled the 

 cavity. Other specimens were more transparent, sometimes exhibiting the beautiful reflections 

 of precious opal. I was able to satisfy myself on the spot, as well as obtain a series of 

 specimens, which show that all these changes, and those of other varieties, which it is not 

 necessary to enumerate, followed the action of hot water containing large quantities of silica in 

 solution upon the porous and permeable structure of tufas and marls. 



Metamorphosis so complete, and due to such a cause, would not be without interest, though 

 limited to small quantities of material. We had here evidences, however, that the metamor- 

 phosis of these tufas extended over a large area, for we found the same or similar changes 

 indicated in the stratified deposits at points on our route more than ten miles distant. Where 

 trap rock had been exposed to the action of these springs, it had, to a great degree, been con- 

 verted into a blood-red pulverulent earth. 



Siiicified ivood is very abundant in the Hot Spring valley, and has doubtless been mineralized 

 by the action of the hot silicious water. I suspect it will be found that the profusion of siiici- 

 fied wood, which has been so frequently noticed in different parts of the area lying between the 

 Cascade range and the Rocky mountains, is traceable to the same cause. 



Fossil wood was also given me, collected near the hot springs at the head of Pit river, by a 

 gentleman whom we saw at Fort Reading, and it is known to abound in those portions of the 

 area called the Great Basin, in New Mexico and northward, which are most marked by volcanic 

 phenomena, and by the occurrence of hot springs. 



At the Cascades of the Columbia, fossil wood has attracted the attention of every traveller who 

 has passed. There, too, I think we can connect its occurrence with recent volcanic eruptions. 



The thermal springs of the Warn Chuck valley are probably of ancient date, and in their 

 origin are doubtless connected with the Warn Chuck mountains, about the base of which 

 they rise. 



WAM CHUCK MOUNTAINS. 



These mountains form a group of rounded summits, rising abruptly from the plain which 

 encircles them on the east, west, and south sides. They are composed of metamorphic slates 

 and trap, exhibiting but little variety of structure or material. They have the appearance of 

 greater age than many of the volcanic hills and ridges which we have passed — an appearance 



