Ch. 7: Quartz Group - Opal, Rock Crystals, Amethysts, Rose Quartz, Agate, etc.

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	136 GEMS AND PRECIOUS STONES IN THE determined by the microscope with the utmost certainty, yet not one particle of the organic matter of the wood remains. It has been entirely replaced by mineral matter, usually some form of silica." The general theory of petrifaction is derived as follows: When wood is soaked in a strong solution of iron sulphate (copperas), then dried, and the same process repeated until the wood is highly charged with this solution and then burned, the structure of the wood will be preserved in the peroxide of iron that remains; also it is well known that the smallest fissures and cavities in rocks are speedily filled by infiltrating waters with mineral matters ; hence wood buried in soil soaked with some petrifying material becomes highly charged with the same and the cells filled with the infiltrating material, so that when the wood decays the petrifying material is left, retaining the structure of the wood. Furthermore, as each particle of organic matter passes away by decay, a particle of mineral matter takes its place, until finally all of the organic matter is replaced. The process of petrifaction is therefore one of substitution as well as of interstitial filling. From the different nature of the process in the two cases, it happens that the interstitial filling always differs, either in chemical composition or in color, from the substituting material. Thus the structure remains visible, although the mass is solid. Prof. James D. Dana offers the following explanation of the phenomenon. "The wood or often trunks of trees, and sometimes standing forests, which have been petrified in the Rocky Mountain region, have in general been buried under volcanic debris, which constitutes beds of great extent in many regions. This volcanic material, called tufa, undergoes partial alteration through the action of the waters or moisture it may contain, or that may filtrate through it. In this alteration or partial decomposition much silica is set free, and makes the waters or moisture silicious. The silicious solution then made penetrates the wood that is buried in the tufa. Very slowly the silica is deposited in all the cells of the wood; and as the wood decomposes, silica takes the place of the particles of the fibres until finally the wood becomes wholly silica or quartz." Concerning the color, he adds that the brownish-yellow is limonite, which if heated will turn red. Among the great

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