514 
THE GEOLOGIST. 
must rest — that geological phenomena must be referred to physical 
causes. 
When the student has obtained sufficient familiarity with these 
established propositions, he will be prepared to follow in the discussion 
of the less elementary problems of the science, many of which are sub- 
jects of controversy. Before, however, going into these speculations, 
Mr. Hopkins devotes several pages to the evidences of the vast antiquity 
of the globe, dwelling both on the evidences afforded by the organic 
remains of successive creations embedded in the rocks, and on those 
drawn from the inanimate kingdom. Of the first we find that an im- 
mense number of different tribes of animals and plants which in former 
ages have given their passing phases to terrestrial life on our planet 
have disappeared with a transition so slow that, according to the pre- 
sent order of nature, there is not the slightest evidence of the introduc- 
tion of a new species of animal since the creation of man ; whilst of the 
latter, it appears certain that the deposition of sedimentary strata, and 
that process of denudation necessarily contemporaneous with it, must 
have proceeded at very much the same rate in former as in recent times ; 
and, admitting this conclusion, we obtain something like a rough con- 
ception of the enormous lapse of time necessary for the deposition of 
the whole mass of sedimentary formations, by simply comparing such 
mass with the proportion which has been transported and deposited 
within the last two or three thousand years. 
Particular cases have thus occasionally been considered with respect 
to the deposition of certain limited masses, as, for instance, the delta of 
the Mississippi, has in this manner been computed at a minimum period 
of some G0,000 years. 
The history of our globe presents to us a continual struggle between 
two antagonistic operations, denudation and elevation. The question 
of the causes of these operations is still matter of controversy, which, 
as well as many other speculations, Mr. Hopkins purposely avoids 
discussing, except so far as it may be necessary to do so in the consid- 
eration of two of the chief speculative questions, to which the study 
of geology naturally leads, and to which the remainder of his essay is 
devoted. 
One of these speculative questions is, "Whether we find distinct evi- 
dence of a progressive change in the physical condition of the globe ; or 
whether such changes as we may recognize in past times are only recur- 
ring j)criodical changes, sometimes taking place, as it were, in one 
direction, sometimes in another, so as not to alter what may be termed 
the mearh condition of the earth, but so as to leave it, during all past 
time, in a physical condition not essentially differing from that in 
which it now exists. 
The progresssive theory is advocated by some of our best geologists ; 
the non-progressive by others of eminence. The advocates of the latter 
are sometimes also designated xmiformitarians. 
The other speculative question relates to the appearance and disap- 
pearance of successive creations or of new forms of animals or vegeta- 
tion, specifically distinct from those which preceded them. Hence, 
" Is the introduction of these new orders of organic beings to be referred 
