a 
MINERALS AND GEOLOGY OF CANADA. 501 
strata, we do not meet with a single trace of the peculiar group of 
Crustaceans termed Trilobites (see figures and description in our 
next Number), although in earlier-formed or lower strata these forms 
occur generally, and often in great abundance. Hence, in a rock 
containing trilobites, no matter how similar such rock may be in as- 
pect and mineral characters to coal-strata, we may be assured that it 
will be useless to bore or excavate for coal, at least with the expecta- 
tion of finding great workable beds of that material, such as occur in 
the proper coal formation. 
Some fossil remains, belonging to the most recent geological de« 
posits, are identical with existing species; others are akin to these, 
without being actually identical with them; and others, again, are 
wholly without representatives in existing Nature. These various 
bodies comprise chiefly: the casts or impressious of sea-weeds, fern- 
fronds, and leaves of higher vegetable types, with occasional fruits 
and stems of trees; the remains of corals, star-fishes, and other 
radiated animals; the shells of mollusca; tests of crustaceans ; and 
teeth, bones, and more or less complete skeletons of vertebrated 
animals. In some cases, these remains have evidently been entombed 
where the plants, corals, mollusks, &c., were actually living ; whilst 
in others, they have been drifted to a greater or less distance with the 
sediments of which they now form part. The process of fossilization 
is a gradual replacement, atom by atom (as in the case of many 
mineral pseudomorphs), of the original organic substance of the body 
by mineral matter. The fossilizing agents comprise the general sub- 
stance of the enclosing sediments, together with certain special sub- 
stances, of which the more common include—silica, carbonate of 
lime, and carbonate of iron, the latter being frequently converted into 
peroxide of iron, and also into iron-pyrites. (See Vol. V., page 171.) 
The causes which principally influence the preservation of organic 
bodies in the fossil slate, comprise : 
1. The habitat of the plant or animal. 
2. The conditions prevailing at the spot to which its remains may 
be brought, or at which it meets its death. 
3. The inherent power of these remains to resist mechanical disin« 
tegration. 
4. Their powers of resistance to chemical decomposition. 
5. The nature of the rock-matters in which they may be enclosed; 
and the after conditions to which these matters may be subjected. 
