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MODE OF OCCUEEENCE OF FOSSIL WOOD. 



237 



ities 



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- 

 — 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 

 Hock Canon, 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, — 



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, 

 !n a manner analogous to that which is not unfrequently observed in 

 minerals and rocks which have undergone or are under^oin^ metainorphic 



a remarkable witness of the great climatic and geo- 



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 preexistence of anything 

 °^ganic 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- 



