James Croll — On the Glacial Epoch. 313 



perfectly inexplicable. It was only when it was found that the ac- 

 cumulated facts refused to be explained by any other conception, that 

 belief in the very existence of such a thing as continental ice became 

 common. But although most geologists now admit the existence of 

 continental ice, yet, nevertheless, adequate conceptions of its real 

 magnitude are by no means so common. Tear by year, as the out- 

 standing facts connected with glaciation accumulate, we are com- 

 pelled to extend our conceptions of the magnitude of land-ice. Take 

 the following as an example. It was found that the transport of the 

 Wastdale Crag blocks, the direction of the striae on the islands of 

 the Baltic, on Caithness and on the Orkney, Shetland, and Faroe 

 islands, the Boulder-clay with broken shells in Caithness, Holderness, 

 and other places, were inexplicable on the theory of land- ice. But 

 it was so only in consequence of the inadequacy of our conceptions 

 of the magnitude of the ice ; for a slight extension of our ideas of its 

 thickness have explained not only these phenomena,^ but others of 

 an equally remarkable character, such as the striation of the Long 

 Island and the submei'ged rock-basins around our coasts described 

 by Mr. James Geikie. In like manner, if we admit the theory of 

 the Glacial epoch propounded in former papers,^ all that is really 

 necessary to account for the submergence of the land is a slight 

 extension of our hitherto preconceived estimate of the thickness of 

 the ice on the Antarctic Continent. If we simply admit a conclusion 

 to which all physical considerations, as we have seen, necessarily 

 lead us, viz. that the Antarctic Continent is covered with a mantle 

 of ice at least two miles in thickness, we have then a complete ex- 

 planation of the cause of the submergence of the land during the 

 Glacial epoch. 



Although of no great importance to the question under considera- 

 tion, it may be remarked that, except during the severest part of the 

 Glacial epoch, we have no reason to believe that the total quantity 

 of ice on the globe was much greater than at present, only it would 

 then be all on one hemisphere. Eemove two miles of ice off the 

 Antarctic Continent, and place it on the northern hemisphere, and 

 this along with the ice that now exists on this hemisphere would 

 equal, in all probability, the quantity existing on our hemisphere 

 during the Glacial epoch ; at least, before it reaches its maximum 

 severity. 



How much, then, would the transference of the two miles of ice 

 from the southern to the northern hemisphere raise the level of the 

 ocean on the latter hemisphere ? This mass, be it observed, is equal 

 to only one-half that represented in our section. A considerable 

 amount of discussion has arisen in regard to the method of deter- 

 mining this point. According to the method detailed in my former 

 papers ("Reader," Sept. 2nd, 1865; Phil. Mag. April, 1866), which 

 supposes the rise at the pole to be equal to the extent of the dis- 

 placement of the earth's centre of gravity, the rise at the North Pole 

 would be about 380 feet, taking into account the effect produced by 



^ See Geol. Mag. for May and June, 1870, and Jan. 1871. 



2 See Phil. Mag. for March, 1870, March, 1874, May, Aug. Nov. 1868. 



