Revieics — Dr. Cr oil's Life and Work. 75 



drift-currents bearing great masses of ice and snow from the Polar 

 regions. 



Apart from all considerations involving abstruse mathematical 

 demonstration, there is one important section of CroU's speculations 

 which must commend itself to those who are desirous of in- 

 vestigating the subject of Climates, past and present. He insisted 

 upon the importance of the warm-water apparatus of the globe in 

 modifying these. We may even believe with him that the " real 

 and effective cause of the disappearance of the ice [of the Glacial 

 epoch] was the enormous transference of heat to temperate and 

 polar regions by means of equatorial currents" (Geological 

 Magazine, 1879, p. 480). True, it is difficult to understand in view 

 of his theory that the colder hemisphere forces the equatorial 

 currents away from itself, how this could be, unless some other and 

 more powerful cause operated to counteract the above-mentioned 

 tendency. This, however, merely refers to causes directing the 

 equatorial currents, and does not affect the general question of their 

 influence. 



The general oceanic circulation is, of course, modified by latitude 

 and certain local causes, and this is especially noticeable in the 

 North Atlantic ; where the steady set of warm surface water towards 

 the north-east, partly initiated by the Florida current, helps to 

 produce such a remarkable contrast between the climate of western 

 Europe and that of corresponding latitudes in eastern America. 

 This difference is evidently one of some antiquity, since the causes 

 whice produce it now were evidently in operation in the Glacial 

 epoch. For the glaciation in eastern America, as was thoroughly 

 realized by Croll, extended some ten degrees further south than 

 it did in western Europe. This fact should be borne in mind by 

 persons who are desirous of attributing the cold of the Glacial 

 epoch to interference with the Gulf Stream, since some of the 

 effects for which that agency is held accountable were certainly 

 produced during the period of Pleistocene glaciation. 



Lastly, we must consider Croll as a geologist, or rather as 

 a glacialist, for he expressly repudiates any knowledge of geology 

 otherwise than in connection with Boulder-clay. In this department 

 he may be said to have done some really practical work by helping 

 to demonstrate the occurrence of important physical features, such 

 as pre-Glacial river-channels, etc. Indeed, his acquaintance with 

 Boulder-clay areas in the Scottish Midlands seems to have been one 

 of his strong points, and he was able to draw important inferences 

 from the knowledge thus obtained. 



Although much that belongs to the history of the Glacial epoch 

 remains as mere hypothesis even to the present day, we owe to the 

 school of which Croll was an eminent member the recognition of 

 the fact that the phenomena of the Boulder-clay and its accessories 

 are in the main due to the action of land-ice. After Jamieson's 

 brilliant explanation of the origin of the parallel roads of Glenroy, the 

 land-ice, or glacier theory, has passed from the region of hypothesis 

 to that of demonstration, and should form the basis of all reasoning 



