64 CAUSES OF GEOLOGICAL CHANGES. 
the surface of the ground, though thrown up before the time of 
Caesar's invasion. The objection was held to be a valid and strong 
one, though as Hutton set no limit to the time during which the 
revolutions he supposed the earth to be undergoing, and to have 
undergone, are accomplished, his partizans would not allow it the 
weight to which it seemed to be entitled. 
A more powerful and decisive objection to the theory of Hutton 
was found in the absence of positive evidence of its truth, so that 
when philosophers were called upon to embrace it, they held it to 
be enough to demand in return the proof 'of the existence of that 
central fire, by which the mineral beds constituting the bottom of 
the sea were to be fused, and the mountains thrown up, and that 
it would operate in the way and produce the effects ascribed to 
it. Of the two rival theories— those of Hutton and Werner, 
neither is at present held in the form under which it was pro- 
posed by its author; but there has unquestionably been a tendency 
in the progress of geological discovery and doctrine, to approach 
that of Hutton, rather than the other. 
CAUSES OF GEOLOGICAL CHANGES. 
37. Before entering upon the subject of theoretical geology, it 
will be necessary to turn our attention to the agents that are now 
modifying the surface of the globe, and enquire what are their 
modes of action, and what the nature and extent of the effects they 
produce. We shall thus be prepared to judge whether the an- 
cient revolutions of the earth bore any intimate resemblance to 
the changes that are proceeding before our eyes, so that we may 
safely attribute the formation of the primitive, transition, and 
secondary strata to the operation of causes that are now active, or 
we are compelled to assume the existence of agents that have now 
disappeared from the face of the globe — or whether again suppos- 
ing the agents the same we must assign to them a strength and 
activity in the most ancient times, very far superior to what is 
exerted by them now. It has been represented that the earth 
as at present, and has been (with a single exception) since the 
era of the creation of man, in a state of comparative repose, the 
convulsions to which it has been subjected since that event, as well 
as the changes now in progress, being inconsiderable when com- 
pared with those of earlier date. A contrary opinion has been 
advanced and advocated with great ability within the last few- 
years — that the amount of geological action and change is much 
the same from age to age: that the. ranges of high mountains 
which traverse the surface of the globe, have not been produced 
by a few violent convulsions, but are the final result of an accu- 
mulation of effects each in itself inconsiderable, and requiring 
together myriads of ages for their accomplishment. Whichever 
of these opinions we embrace, a knowledge of the present will be 
