514 



THE GEOLOGIST. 



must rest — that geological phenomeiia 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 creatiocs 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 60,000 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 periodical changes, sometimes taking place, as it were, in one 

 direction, sometimes in another, so as not to alter what may be termed 

 the mean 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 pro(jresssive theory is advocated by some of our best geologists ; 

 the non-jmjgressive by others of eminence. The advocates of the latter 

 are' sometimes also designated iiniformitarians. 



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 



