Febbuaby 1, 1907] 



SCIENCE 



169 



of boulders by icebergs held the ground 

 longer. In 1864 Lyell still maintained the 

 submergence of the plain of northern Eu- 

 rope, and of the glaciated region of North 

 America; and it was not until 1875 that 

 Torell, in a memorable meeting of the Ger- 

 man Geological Society, convinced the 

 German geologists that the drift of north- 

 ern Germany was transported by an ice 

 sheet whose center was in the mountains 

 of Scandinavia, and the name Diluvium, 

 though still used in German geological 

 writings, was completely emptied of its 

 original connotation. 



"Within the last few decades the labors 

 of a large number of earnest and able in- 

 vestigators have developed the glacier the- 

 ory more in detail, and have added vastly 

 to our knowledge of Quaternary history. 

 The imaginary polar ice cap has given 

 place to ice sheets of more limited dimen- 

 sions, though still vast, developed respect- 

 ively about the Laurentide, Keewatin and 

 Cordilleran centers. The series of terminal 

 moraines, marking stages of readvanee or 

 halts in the retreat of the ice sheet, have 

 been carefully mapped. From the experi- 

 ence gained in the study of terminal mo- 

 raines in this country, Lewis was enabled, 

 in 1886, to recognize and interpret the 

 terminal moraines of the ice sheet in Eng- 

 land ; and Salisbury, in the following year, 

 those of north Germany. The recognition 

 of interlobate moraines and of driftless 

 areas led to a clearer understanding of the 

 nature of the movement of the great ice 

 sheet. The driftless area of Wisconsin was 

 noticed by J. D. "Whitney as early as 1862, 

 and has been more carefully studied by 

 Chamberlin and Salisbury; and the latter 

 investigator has called attention to a small- 

 er driftless area in Illinois. The Quater- 

 nary period, instead of being brief and 

 comparatively simple, has been shown to 

 be of long duration and great complexity. 

 It has been analyzed into a succession of 



glacial and interglacial epochs; and, from 

 the vast amount of erosion in some of the 

 interglacial epochs, it has been inferred 

 that post-Glacial time is very short in com- 

 parison with inter-Glacial time. The his- 

 tory of the series of lakes held between the 

 front of the receding ice sheet and the 

 southern water shed of the Saint Lawrence 

 basin, has been studied by Gilbert, Taylor, 

 Fairehild and others. In Chamberlin 's 

 theory that the cause of the Glacial climate 

 is primarily the diminution of the amount 

 of carbon dioxide in the atmosphere, and 

 that the location of the main centers of 

 glaciation is due to the path of cyclonic 

 storms, we have a theory which, if it can 

 not be accepted with full confidence as the 

 true one, is at least the only theory of the 

 Glacial climate which has not yet been 

 weighed in the balance and found wanting. 

 There seems still to be some disagreement 

 among physicists on the critical question, 

 what effect small changes in the amount of 

 carbon dioxide in the atmosphere would 

 produce upon the climate. The views of 

 Arrhenius, upon which the theory of 

 Chamberlin is based, have been contested 

 by Angstrom and Very, and are not ac- 

 cepted by Hann. If the physicists can be 

 brought to an agreement on this funda- 

 mental point, we may feel that we have, at 

 last, a theory of the Glacial climate. The 

 alternative at present seems to be the ac- 

 ceptance of Chamberlin 's theory, or the 

 confession that we have as yet no explana- 

 tion of the Glacial climate. 



IV. SUBAEEIAL DENUDATION AND THE EVO- 

 LUTION OF DRAINAGE SYSTEMS 



In early years the study of geology in 

 this country was substantially confined to 

 the region east of the Mississippi; but, in 

 due season, the weird and fascinating re- 

 gion of the Cordillera revealed itself to 

 explorers and geologists. It is now more 

 than half a century since American geolo- 



