330 



SCIENCE 



[N. S. Vol. XXXII. No. 819 



tions of the Alpine valleys were in the 

 main excavated during Pliocene ages, their 

 upper and more open parts being largely 

 the results of Miocene and yet earlier 

 sculpture. 



During the great advances of the ice, 

 four in number, according to Penck and 

 Briickner,^' when the Rhone glacier cov- 

 ered the lowlands of Vaud and Geneva, 

 welling on one occasion over the gaps in 

 the Jura, and leaving its erratics in the 

 neighborhood of Lyons, it ought to have 

 given signs of its erosive no less than of its 

 transporting power. But what are the 

 facts ? In these lowlands we can see where 

 the ice has passed over the Molasse (a 

 Miocene sandstone) ; but here, instead of 

 having crushed, torn and uprooted the com- 

 paratively soft rock, it has produced 

 hardly any effect. The huge glacier from 

 the Linth Valley crept for not a few miles 

 over a floor of stratified gravels, on which, 

 some eight miles below Zurich, one of its 

 moraines, formed during the last retreat, 

 can be seen resting, without having pro- 

 duced more than a slight superficial dis- 

 turbance. "We are asked to credit glaciers 

 with the erosion of deep valleys and the 

 excavation of great lakes, and yet, where- 

 ever we pass from the hypotheses to facts, 

 we find them to have been singularly in- 

 efficient workmen! 



I have dwelt at considerable, some may 

 think undue, length on the Alps, because 

 we are sure that this region from before 

 the close of the IMioeene period has been 

 above the sea-level. It accordingly dem- 

 onstrates what effects ice can produce 

 when working on land. 



In America also, to which I must now 

 make only a passing reference, great ice- 

 sheets formerly existed : one occupying the 

 district west of the Rockj^ Mountains, 



" On the exact number I have not had the op- 

 portunity of forming an opinion. 



another spreading from that on the north- 

 west of Hudson's Bay, and a third from 

 the Laurentian hill-country. These two 

 became confluent, and their united ice-flow 

 covered the region of the Great Lakes, 

 halting near the eastern coast a little 

 south of New York, but in Ohio, Indiana 

 and Illinois occasionally leaving moraines 

 only a little north of the 39th parallel of 

 latitude.'^' Of these relics my first-hand 

 knowledge is very small, but the admirably 

 illustrated reports and other writings of 

 American geologists^^ indicate that, if we 

 make due allowance for the differences in 

 environment, the tills and associated de- 

 posits on their continent are similar in 

 character to those of the Alps.-" 



In our own country and in corresponding- 

 parts of northern Europe we must take 

 into account the possible cooperation of the 

 sea. In these, however, geologists agree 

 that, for at least a portion of the ice age, 

 glaciers occupied the mountain districts. 

 Here ice-worn rocks, moraines and perched 

 blocks, tarns in corries, and perhaps lake- 

 lets in valleys, demonstrate the former- 

 presence of a mantle of snow and ice. 

 Glaciers radiated outwards from more 

 than one focus in Ireland, Scotland, the 

 English Lake District, and Wales, and 

 trespassed, at the time of their greatest de- 

 velopment, upon the adjacent lowlands. 

 They are generally believed to have ad- 

 vanced and retreated more than once, and 



" Some of the glacial drifts on the eastern side 

 of the continent, as -we shall find, may have been 

 deposited in the sea. 



" See the Reports of the United States Geolog- 

 ical Survey (from Vol. III. on-svards), Journal of 

 Geology, American Journal of Science, and local 

 publications too numerous to mention. Among 

 these the studies in Greenland by Professor Cham- 

 berlin are especially valuable for the light they 

 tbrow on the movement of large glaciers and the 

 transport of debris in the lo-wer part of the ice. 



^ Here, ho-n-ever, -we can not al-n-ays be so sure 

 of the absence of the sea. 



