REPORT ON GEOLOGY. 
411 
from the contemporaneous action of similar causes in distant 
localities; in each of which many varying local circumstances 
must have affected those results; for two conditions obviously 
enter into this problem :—first, the contemporaneous preva¬ 
lence and extent of similar geological causes; and secondly, 
how far these causes, even where active, may have been modi¬ 
fied by varying local circumstances. Now, at present, our ma¬ 
terials for answering these questions accurately are confined 
to Europe: of America indeed we have some information; and 
although this may as yet be considered too vague to be fully 
satisfactory, yet as far as it goes it is undoubtedly favourable 
to the presumption of even a greater degree of geological uni¬ 
formity, than we should have been justified in anticipating a 
priori. 
Humboldt indeed has remarked, that while on entering a 
new hemisphere we change all other familiar and accustomed 
objects,—while in the plains around we survey entirely new 
forms of vegetable and animal being, and in the heavens over 
our heads we gaze on new constellations,—in the rocks under 
our feet, alone, we recognise our old acquaintances. And with 
regard to the primordial rocks, there is undoubtedly much 
truth in this pointed remark. Granite, mica slate, and their 
contained minerals, present the most identical resemblance, 
whether we collect from Dauphine, Norway, the Alleghanies, 
Egypt, India or Australia. But concerning the secondary series 
our information is far less precise. 
With regard to the comparative geology of secondary di¬ 
stricts, the fossil zoology of the various districts, or compara¬ 
tive p alee ontology, requires to be called to our aid as our surest 
guide. And these investigations are the more interesting, because 
an important primary question here suggests itself. In the ac¬ 
tual state of things, the limited geographical distribution of iden¬ 
tical species, both animal and vegetable, is one of the most strik¬ 
ing phenomena that presents itself to our view. In distant con¬ 
tinents, the specific differences of the animal races are wide and 
strongly marked ; and sometimes, as in Australia, the difference 
extends to the character of genera, and even families; and 
this even in countries of similar conditions, as to latitude, cli¬ 
mate, and temperature. Now we may naturally inquire, whether 
it does not seem most probable, that in the ancient geological 
aeras the species then inhabiting our globe were grouped to¬ 
gether under similar restrictions as to geographical habitation; 
and if so, how far we are entitled to expect to find in other 
countries the same series of successive organic remains, (each 
group characterizing a distinct geological aera,) which we meet 
with in Europe, To illustrate this inquiry :—If in the present 
