APPENDIX. 



285 



sub- 

 illed, 



and then — the walls or solid parts being removed by decay or solu- 

 tion — mineral matter, either similar to that filling the cavities, or 

 ditroring in colour or composition, has been introduced. Silicifled 

 wood often occurs in this condition. In the case of silicifled wood, 

 it sometimes hapi)cns that tlie cavities of the fibers have boon filled 

 with silica, and the wood has been afterward removed l»y decay, 

 leaving the casts of the tubular fibers as a loose filamentous sub- 

 stance. Some of the Tertiary coniferous woods of California are in 

 this state, and 'ook like asbcstus, though they show the minute 

 markings of the tissue under the microscope. In the case of silicilied 

 or agatized woods, it would seem that the production of carbon di- 

 oxide from the decaying wood has caused the deposition of silica in 

 its place, from alkaiina solutions of that substance, and tlius the 

 carbon has been replaced, atom by atom, by silicon, until the whole 

 mass has been silicifled, yet retaining perfectly its structure. 



(c) The cavities left by fossils which have decayed may be fllled 

 with clay, sand, or other foreign matter, and this, becoming subse- 

 quently hardened into stone, nuiy constitute a cast of the fossils. 

 Trunks of trees, roots, &c., arc often preserved in this way, appearing 

 as stony casts, often with the outer bark of the plant forming a car- 

 bonaceous coating on their surfaces. In connection with this state 

 may be mentioned that in which, the wood having decayed, an entire 

 trunk has been flattened so as to appear merely as a compressed film 

 of bark, yet retaining its markings ; and that in which the whole of 

 the vegetable nuitter having been removed, a mere impression of 

 the form remains. 



Fossils preserved in either of the modes, (a) or (b), usually show 

 more or less of their minute structures under the microscope. These 

 may be observed : — (1) By breaking off small splinters or flakes and 

 examining them, either as opaque or as transparent objects. (2) By 

 treating the material with acids, so as to dissolve out the mineral 

 matters, or portions of them. This method is especially applicable 

 to fossil woods mineralised with calcite or pyrite. (3) By grinding 

 thin sections. These arc flrst polished on one face on a coarse stone 

 or emery hone, and then on a fine hone, then attached by the polished 

 face to glass slips with a transparent cement or Canada balsam, and 

 ground on the opposite face until they become so thin as to be trans- 

 lucent. In most cities there arc lapidaries who propiire slices of this 

 kind; but tlie amateur can readily acipiire the art by a little prac- 

 tice, and the necessary appliances can be obtained through dealers 

 in minerals or in microscop'i materials. Very convenient cutting 

 and polishing machines, some of them quite small and portable, are 

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