4° 



GLACIERS 



altered and scattered deposits which form the rocks of the 

 earth's crust. 



The most striking fact which the investigation of this 

 subject has brought into prominence is this : The exten- 

 sion of the glaciers even to the limit which was reached in 

 the last great glacial period of prehistoric times does not 

 imply any very extreme climate. That is to say, a small 

 change in our present climate would bring back the exten- 

 sion of glaciers, and give us another glacial period. Glaciers 

 require heat as well as cold — heat to raise the vapour from 

 adjacent regions, which is then condensed as snow on the 

 higher and colder territory. The presence of an excess of 

 aqueous vapour in the atmosphere, even without the forma- 

 tion of cloud, has a very large and important result in 

 stopping the access of heat from the sun to the region of 

 the earth protected by the moist atmosphere. An increase 

 of watery vapour in the atmosphere of the northern hemi- 

 sphere would materially lower its temperature. A succes- 

 sion of damp summers would do more to enlarge the 

 glaciers than a series of severe winters. It is estimated by 

 competent authorities that a fall in the average annual 

 temperature of only io° F. (provided that the summers 

 were cold and damp) would suffice in a few decades to 

 bring about the return of glacial conditions in the 

 temperate region of the northern hemisphere. The 

 present snow-line, or level of perpetual snow, is, in this 

 part of the world, at the height of 9000 ft. above sea- 

 level. As we ascend from the sea-level the temperature 

 decreases by about 1° F. for every 350 ft. we mount. A 

 fall of 10° in temperature would accordingly bring the 

 snow-line down by 3500 ft. — that is, to S 500 ft. above sea- 

 level — and the lower limit to which the glaciers reach, 

 which is now about 4000 ft. above sea-level, would 

 descend to 1200 ft, which is lower than the level of 

 Geneva. Changes in the outline of the continents and in 



