MODE OF OCCURRENCE OF FOSSIL WOOD. 237 
matter. Specimens of the almost colorless opalized wood do sometimes, it is 
true, blacken on exposure to heat, from the presence of organic matter, no 
doubt, and the naturally black portions do not burn white when exposed to 
the blowpipe flame. The carbon seems to be too thoroughly enveloped in the 
silicious material. In one remarkable instance the body of the dark-colored 
silicified wood was observed to be penetrated by large and well-formed quartz 
crystals, from a third to half an inch in diameter. These crystals, on ex- 
amination with the microscope, proved to be partly chalcedonic in structure, 
the exterior or border, which was sharply defined on the outer edge, being 
chiefly quartz. The process of silicification has gone on, in the gravel and 
accompanying volcanic beds, on the most extraordinary scale. In some local- 
ities — as, for instance, at Chalk Bluffs — one walks over the hydraulicked 
area among the fallen silicified logs, which lie scattered pell-mell, as if they 
had been prostrated by a tornado. The well-known “ petrified forest,” in the 
Coast Ranges, near Calistoga, is another good instance of the same thing. 
In the latter case the silicified trees are imbedded in a dark andesitic ash ; 
in the former, the enveloping material is mostly a fine rhyolitic tufaceous 
mass. At a locality in Nevada, near the California border, not far from Black 
Rock Caiion, a large silicified log was observed by Mr. C. F. Hoffmann, resting 
on the surface, in a region where at present there is no forest growth at all. 
This log was four or five feet in diameter, and more than a hundred feet of 
it was exposed to view, —a remarkable witness of the great climatic and geo- 
logical changes which have gone on in this region during the latest epoch. 
The most striking kind of fossil wood occurring in the mining region is the 
fibrous variety, of which beautiful specimens are occasionally found. Some 
of these have the most delicately fibrous structure, somewhat resembling 
raw silk both in texture and in color. The fibres are often very long and 
straight, like those of the finest asbestus. This peculiarity evidently has 
nothing to do with the original structure of the wood. The fibres have been 
formed, in the process of the replacement of the ligneous particles by silica, 
in a manner analogous to that which is not unfrequently observed in 
minerals and rocks which have undergone or are undergoing metamorphic 
changes. Chrysolite, the fibrous variety of serpentine, and asbestus, a similar 
form of hornblende, are good instances of this kind of structure in simple 
minerals, where there is no reason to suppose the preéxistence of anything 
organic to bring about this peculiar arrangement of the particles. 
It is not rare to find that portions of the wood occurring in the gravel de- 
