44 BULLETIN OF THE 



gate effect. This astronomical fact presents at least a vera causa, 

 which unquestionably must produce some secular change of cli- 

 mate. 



On the other hand, the "geographical" theory so ably urged 

 by Lyell is essentially speculative. We know indeed that great 

 and repeated changes of elevation have occurred in the land, bat 

 that they have occurred in the directions required by the theory 

 is pure assumption. There is good reason to believe that no 

 considerable change in the positions of the continents has taken 

 place since the commencement of the glacial epoch. 



Years before Mr. Croll advanced his theory, there was a grow- 

 ing opinion among several leading geologists, such as Gumming, 

 Goodwin, Austen, Ramsay, Page, and others, that we had indi- 

 cations of colder periods of long continuance, in the Mesosoic and 

 Palaeozoic ages ; notably in the Cretaceous, and in the Permian, 

 the Devonian, and even in the remote Cambrian formations. 

 Such recurrences of what may be called glacial epochs, would of 

 themselves seem to point rather to cosmical, than to local or geo- 

 graphical causes. Any considerable change in the latitude dis- 

 tribution of land and water would, of course, have its effect on 

 the general local climate ; but from the above point of view, 

 would have to be regarded rather as a perturbing, than as an 

 originating influence. 



Mr. Powell continued the discussion, objecting to Croll's 

 theory, that it unfortunately gave fixed dates, or determinate 

 periods, which were really as inadequate (in the view of practical 

 geologists) for the work accomplished and the changes effected, as 

 was the earlier Mosaic chronology. Mr. P. then gave an account 

 of his observations on the character and extent of the erosion ex- 

 hibited in our western river beds, and on the various distributions 

 of gravels, as furnishing irresistible evidence of the requirement 

 of far longer periods of time than had ordinarily been assigned 

 for the formation of such conglomerates. 



