40, MINERALIZATION OF [Ch. IV. 



hibited. A texture equally minute and complicated has been observed 

 in the wood of large trunks of fossil trees found 

 in the Craigleith quarry near Edinburgh, where 

 the stone was not in the slightest degree siliceous, 

 but consisted chiefly of carbonate of lime, with 

 oxide of iron, alumina, and carbon. The parallel 

 rows of vessels here seen are the rings of an- 

 nual growth, but in one part they are imperfectly 

 preserved, the wood having probably decayed 



coal strata, magnified. (Wi- before the mineralizing; matter had penetrated to 



tham.) Transverse section. ° x 



that portion of . the tree. 



In attempting to explain the process of petrifaction in such cases, we 

 may first assume that strata are very generally permeated by water 

 charged with minute portions of calcareous, siliceous, and other earths 

 in solution. In what manner they become so impregnated will be after- 

 wards considered. If an organic substance is exposed in the open air 

 to the action of the sun and rain, it will in time putrefy, or be dissolved 

 into its component elements, which consist chiefly of oxygen, hydrogen, 

 and carbon. These will readily be absorbed by the atmosphere or be 

 washed away by rain, so that all vestiges of the dead animal or plant 

 disappear. But if the same substances be submerged in water, they de- 

 compose more gradually ; and if buried in earth, still more slowly, as in 

 the familiar example of wooden piles or other buried timber. Now, if 

 as fast as each particle is set free by putrefaction in a fluid or gaseous 

 state, a particle equally minute of carbonate of lime, flint, or other min- 

 eral, is at hand and ready to be precipitated, we may imagine this inor- 

 ganic matter to take the place just before left unoccupied by the organic 

 molecule. In this manner a cast of the interior of certain vessels may 

 first be taken, and afterwards the more solid walls of the same may 

 decay and suffer a like transmutation. Yet when the whole is lapidified, 

 it may not form one homogeneous mass of stone or metal. Some of the 

 original ligneous, osseous, or other organic elements may remain mingled 

 in certain parts, or the lapidifying substance itself may be differently 

 colored at different times, or so crystallized as to reflect light differ- 

 ently, and thus the texture of the original body may be faithfully 

 exhibited. 



The student may perhaps ask whether, on chemical principles, we have 

 any ground to expect that mineral matter will be thrown down precisely 

 in those spots where organic decomposition is in progress 1 The following 

 curious experiments may serve to illustrate this point. Professor Gop- 

 pert of Breslau attempted recently to imitate the natural process of pet- 

 rifaction. For this purpose he steeped a variety of animal and vegetable 

 substances in waters, some holding siliceous, others calcareous, others 

 metallic matter in solution. He found that in the period of a few weeks, 

 or even days, the organic bodies thus immersed were mineralized to a 

 certain extent. Thus, for example, thin vertical slices of deal, taken 

 from the Scotch fir (Pinus sylvestris), were immersed in a moderately 



