70 
THE GEOLOGIST. 
some well-knowii 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-knowa 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, 
putrefaction or decomposition, is often most beautifully preserved in petrified 
specimens of woods. 
