in Nexo Countries. 
7 
necessary to attend to these characters, to note of what 
material any rock is composed ; but no inference can legi¬ 
timately be drawn from the nature of that material, as to 
the age of the rock, or its place in the geological series. 
Whether the rock be hard slate or soft clay, compact 
quartz rock or loose sand, it is unsafe to draw any con¬ 
clusion whatever as to its ae;e from these circumstances. 
The next character of aqueous rocks, and a much more 
valuable one, is derived from the organic remains con¬ 
tained in them. From a few fossil shells the age of the 
rock, or its place in the series, may often be at once ascer¬ 
tained within certain limits. In applying this test, how¬ 
ever, much caution is necessary. In the first place we 
have not arrived, and probably never can arrive, at a full 
knowledge of the conditions, at any one time, of animal 
or vegetable life upon the globe. Owing merely to local 
circumstances, one set of animals or plants may have flour¬ 
ished in one place and another at another during the same 
time, or one race may have survived some time under 
favourable circumstances in one place, after it became 
extinct in another. The difference in the organic remains, 
therefore, in the rocks of two places may be no evidence 
of a difference in their periods of deposition. Again, we 
know that, at the present time, the opposite hemispheres 
of the globe are peopled both by animals and plants 
almost entirely specifically distinct from each other. A 
priori , therefore, we have reason to conclude that such 
was the case in old time; and, until experience may show 
us the contrary, we have no reason to expect to find in the 
rocks of Australasia, for instance, any one fossil specifi¬ 
cally identical with one of Europe. While, however, we 
acknowledge the probable necessity for great caution in 
order to avoid mistakes from these causes, long-con¬ 
tinued observation and wide-spread research will always 
eliminate error from our ultimate result. In the first 
