Vol. 58.] CONDITIONS OF THE PLEISTOCENE EPOCH, 39 
only the glaciation of the low land is considered). Then, too, most 
geologists agree in considering that a lowering of the preseut snow- 
line to the extent of about 3300 feet, corresponding to a lowering 
of the mean annual temperature of the earth’s surface by 7° or 
9° Fahr., was the general cause of the Great Ice-Age. 
But it must be borne in mind that glaciation, when once begun, 
deteriorates the climate, as it increases the snowfall and lowers the 
summer-temperature ; and that the ice-sheet at the edges is main- 
tained by the ice-streams coming from the central parts, and not by 
the snow falling at the edges, where much more ice is thawing than 
is falling in the form of snow. Inthis manner the glaciation 
ofthe British Isles may be explained: it was probably 
due merely to the circumstance that the centre of 
glaciation in Scandinavia sent its ice-streams across 
the North Sea, and thereby modified the climate of the British 
Isles. 
The hypothesis that all this glaciation was caused by an upheaval 
of the ice-covered districts from 3000 to 5000 feet, is in itself very 
improbable, as the phenomenon of increased glaciation took place 
nearly all over the earth’s surface. Such an hypothesis is certainly 
not established by means of geological facts, but imagined only in 
order to explain a glaciation for which no other sufficient cause 
could be found. If this explanation were true, the melting-away 
of the ice must also be explained in a similar way, that is, by an 
equally great subsidence. Now this melting-away, according to 
Baron Gerard De Geer, went on very rapidly in the centre of 
Kastern Sweden (plain of Upland), namely, at a rate of 300 feet 
or more every year horizontally (along the ground), whereas no 
simultaneous subsidence of corresponding dimensions took place. 
By this I do not mean to deny that great alterations in level 
took place before, during, and after the Great Ice-Age, but I do 
affirm that the phenomenon of glaciation as a whole was 
not controlled by them. 
The hypothesis that a glaciation of North America would raise 
the temperature of Europe, and vice versd, by means of an alteration 
of the great centres of action of atmospheric circulation seems to 
me to be physically untenable. For the influence of the Atlantic 
continuation of the Gulf-Stream, and the south-westerly winds 
generated by it, on the climate of Europe, would be powerless to 
prevent an ice-age, if a general lowering of the mean temperature 
of the earth’s surface amounting to 7° or 9° Fahr. took place. In 
fact, that influence has rather a cooling effect during the summer, 
because the temperature of the European continent, and also that of 
the British Isles, is then higher than that of the Atlantic. More- 
over, the temperature of those Isles is, in July, somewhat lower than 
the average for the corresponding parallel. Only during the winter 
has the Atlantic any very considerable influence in warming North- 
western Europe. But as only the warmth of the summer can 
prevent glaciation, it is evident that, so soon as snow falls in the 
