16 THE MEDALS OF OREATION. Crap. I. 
many years, or even centuries ; and when deeply imbedded 
in the earth, protected from atmospheric influences, and 
subjected to the conservative effects of various mineral 
solutions, the most perishable tissues often resist decom- 
position, and becoming transformed into stone, may endure 
for incalculable periods of time. The calcareous and sili- 
ceous cases or frustules of numerous microscopic plants are 
so indestructible, and occur in such inconceivable quanti- 
ties, that the belief of some eminent naturalists of the 
last century, that every grain of flint and lime in certain 
rocks, may have been elaborated by the energies of vitality, 
can no longer be regarded as an extravagant hypothesis. 
Some idea may be formed of the large proportion of the 
solid materials of the globe that has unquestionably ori- 
ginated from this source, by a reference to the list of strata 
which are wholly, or in great part, composed of animal 
and vegetable structures, given in the “ Wonders of 
Geology,” p. 888. 
There are also immense tracts of country that consist 
in a great measure of the remains of plants in the state of 
anthracite, coal, lignite, &c.; and districts covered with 
peat-bogs and subterranean forests. 
Although these relics of animal and vegetable organisms 
are found in almost every sedimentary deposit, yet they 
occur far more abundantly, and in a better state of preser- 
vation, in some strata than in others: nor are they equally 
distributed throughout the same bed, but are heaped toge- 
ther in particular localities, and occur but sparingly, or are 
altogether absent, in other layers of the same rock. Neither 
are the remains of the same kinds of animals and plants 
found indiscriminately in strata of different ages: on the 
contrary, many species are restricted to the most ancient, 
others to the most recent formations ; while some genera 
range through the entire series of deposits, and also appear 
as denizens of the existing seas. Hence organic remains 
