396 Dr. James Croll — On Geological Time. 



as a rule, not by eruptions and upheavals, but by the slow process 

 of subaerial denudation. 



Cataclj'smic explanations of phenomena have to a large extent 

 disappeared from the field of physical geology. But there is one 

 department in which they still monopolize the field; viz. in that 

 which treats of great climatic changes in former ages. Just as in 

 physical geology great and imposing effects have been attributed to 

 extraordinary causes, so in questions of geological climate vast vicissi- 

 tudes have been referred to equally vast and unusual agencies. 



We know that at a period comparatively recent almost the entire 

 Northern hemisphere down to tolerably low latitudes was buried 

 under snow and ice, the climate being perhaps as rigorous as that of 

 Gi'eenland at the present day. And we know further that at other 

 periods, Greenland and the Arctic regions were not only to a large 

 extent at least free from ice, but also enjoyed a climate as warm and 

 genial as that of England. To attribute results so striking and 

 stupendous to such commonplace agencies as ocean currents, winds, 

 clouds, and aqueous vapour is at present considered to be little else 

 than absurd. Extraordinary and imposing causes proportionate to 

 the effects are therefore sought. 



To account for the Glacial Epoch, for example, the land was at one 

 time supposed to have stood much higher than at present. It was 

 soon discovered, however, that the glaciation was much too general 

 to be explained by such means. Others believed that it might be 

 accounted for by assuming a displacement of the continents, but this 

 hypothesis had likewise to be abandoned when it became known that 

 no alteration in the position of our continents and ocean basins has 

 taken place since the Glacial Epoch. 



Others again imagined that some great change had probably taken 

 place in the obliquity of the ecliptic so as to bring the Arctic circle 

 down to beyond the latitude of England. And in order to bring 

 this about what enormous upheavals were supposed to have oc- 

 curred! It was soon, however, shown that no possible rearrangement 

 of matter on our globe could materially affect the obliquity ; and 

 besides this, it was further pointed out that even supposing the 

 Arctic circle was by such means to be shifted down to our latitude, 

 yet it would not bring an Arctic climate along with it but the reverse. 

 This hypothesis being in its turn abandoned, it was next assumed 

 that the earth's axis of rotation must have been moved so as to carry 

 our island up to the Arctic regions. But to shift the axis of rotation 

 even so much as 3°, upheavals and subsidences of a magnitude 

 hitherto unheard of in geological speculations had to be assumed. 

 A change of 3°, however, being totally inadequate to account for the 

 great changes of climate in question, earthquakes of sufficient power 

 to break up the solid framework of the globe had to be called into 

 opei-ation, so as to cause a reari'angement of matter sufficient to 

 produce a displacement of the pole to the extent required. The 

 amount of distortion necessitated by this theory is so enormous that 

 most of its advocates have recently abandoned it as hopeless. 



But is there really after all any necessity for invoking the aid of 



