710 THE WONDERS OF GEOLOGY. Lect. VII. 



was stained black, thus indicating that the colour of the 

 coal shales is from the carbon derived from the plants they 

 include. Plants soaked in a solution of sulphate of iron 

 were dried and heated till every trace of organic matter 

 had disappeared, and the oxide was found to present the 

 form of the plant. In a slice of pine-tree the punctured 

 vessels peculiar to this family of vegetables were percep- 

 tible. These results by heat are probably produced natu- 

 rally, by the action of moisture under great pressure, and 

 the influence of a high temperature. 



32. SlLICIFICATION, OR PETRIFACTION OF VEGETABLES 



by silex. — The various forms in which silex is found, are 

 proved to have been dependent on its state of solution ; in 

 quartz crystals it was entirely dissolved; in agate and chal- 

 cedony it was in a gelatinous state, assuming a spheroidal, 

 or orbicular disposition, according to the motion given to 

 its molecules. Its condition appears also to have been 

 modified by the influence of organic matter. In some 

 polished slices of flints from Bognor, the transition from 

 flint to agate, chalcedony, and crystallized quartz, is beau- 

 tifully exhibited. The shell of an echinus, in my posses- 

 sion, is transmuted into crystallized carbonate of lime, while 

 the lower portion of the cavity is occupied by flint, on 

 which is a layer of crystals of calcareous spar. The curious 

 fact, that the shells of the echinites in the chalk are almost 

 invariably filled with flint, while the crustaceous covering 

 is converted into calcareous spar, is, perhaps, attributable 

 to the animal matter having undergone silicification ; for the 

 most organized parts are those which appear to have been 

 most susceptible of siliceous petrifaction. In a specimen in 

 my collection, the oyster itself is turned into flint, while the 

 shell is, as usual, carbonate of lime.* The shells of mol- 

 lusca, the crustaceous skeletons of echinoderms, and the 

 guards of the belemnites, appear to have possessed too 

 * Medals of Creation, p. 363. 



