70 THE GEOLOGIST. 



some well-known plant transformed into a hard stone, and yet 

 retaining most of its colour and external appearance ? Let us see how 

 this happens. In one of the paragraphs of our last article we had 

 occasion to notice the action of water upon the so-called Plutonic rocks. 

 We have seen that an alcaline silicate is invariably dissolved out of the 

 rock by water. In nature this silicate is carried away by the rain- 

 water to fertilize the earth ; it is absorbed with the water by plants, 

 many of which rob it of its silica in an extraordinary manner. Vege- 

 tables are known to evolve in the act of respiration, and more 

 especially at night, a certain quantity of carbonic acid. We know 

 also that this acid decomposes silicate of potash or soda, and in a few 

 hours precipitates the silica of these salts. It is in this way that the 

 different grasses and the plants of the genus Equisetum, &c., retain in 

 their tissues considerable quantities of silica, especially in the green 

 parts of the epidermis or cuticle, where the function of respiration is 

 most active.* But when a piece of wood undergoes decomposition, 

 and becomes brown and decayed, it also evolves a great quantity of 

 carbonic acid, which acts on the alcaline silicates with which the wood 

 is often imbued by the absorption of spring water, in the way we have 

 just described, and precipitates their silica. In this manner, for every 

 atom of carbonic acid furnished by the carbon of the decaying vege- 

 table, an atom of silica is substituted ; so that, after a certain space of 

 time, the entire tissue of the plant is converted into a tissue of hard 

 stone, f Animal substances, such as the bodies of polypes, mollusca, 

 &c., furnish principally carbonate of ammonia by decomposition. This 

 volatile salt acts upon the silicates of potash and soda in the same way 

 as carbonic acid, explaining to us at once the manner in which the 

 petrifaction of shell-fish and other animals takes place. 



But to return to M. Kuhlman — the most important part of his memoir 

 relates to epigenic forms (or the covering over or replacing of one 

 mineral species by another, without change of form). He has en- 



• It is a well-known fact that certain sugar-canes, bamboos, &c., will often 

 strike fire with steel, on account of the large quantity of silica contained in the 

 bark of their stems. 



t The colouring matter of the bark of the trees being accompanied by tannic 

 acid and other substances which have the faculty of preventing, in a great degree, 

 P'lt refaction or decomposition, is often most beautifully preserved in petrified 

 t^pccimens of woods. 



