294 FOSSIL GEOLOGY OF UNITED STATES. 
In forming a correct idea of the mode of petrifac- 
tion, we must bear in mind that rock strata are very 
generally permeated by water charged with minute 
portions of calcareous, silicious, and other earths 
in solution ; for we have already seen that water is 
capable of dissolving mineral substances to a con- 
siderable extent. When an animal or vegetable is 
left exposed to the open air, it putrefies and is de- 
composed into its original elements, oxygen, hydro- 
gen, and carbon, which escape into the air or are 
washed away by the rain, so that all vestiges of 
tliem shortly disappear. Suppose, now, that they 
aie buried in the earth, they will decompose, of 
cc'urse, more gradually; and, as fast as each particle 
is set free by decomposition, its place will be filled 
with particles of mineral matter, such as lime, or 
flint, or clay. In this way a cast of the interior of 
certain vessels may be taken, and afterward the 
walls of the same may decay and sulFer a like trans- 
mutation. But when the whole is changed into 
stone, it may not form one homogeneous mass of 
stone or metal ; for either some of the original 
woody or bony matter may remain mingled in cer- 
tain parts, or, what is more often the case, the min- 
eral which is deposited may be so crystallized in 
different parts as to reflect light difi'erently, and 
thus exhibit the exact texture of the original body. 
For example, Prof. Goppert, of Breslau, steeped a 
variety of animal and vegetable substances in water, 
some holding silicious, others calcareous, others 
metallic matter in solution. He found that in the 
period of a few weeks, or even days, the original 
bodies thus immersed were mineralized to a certain 
extent. Thus, then, vertical slices of deal, taken 
from the Scotch fir, w^ere immersed in a moder- 
ately strong solution of sulphate of iron. When 
they had been thoroughly soaked in the liquid for 
several days, they were dried and exposed to a red 
heat until the vegetable matter was burned up, and 
