AMEKICAN GEOLOGY — DECADE OF 1870-1879. 581 



place an increased evaporation from the land and water was necessary. 

 This could be brought about only byageneral increase of temperature, 

 the amount of precipitation being the same; whether it would or not 

 give rise to glaciers would depend upon temperature, which itself 

 would be dependent upon local conditions which might or might not be 

 due to elevation of land surfaces. His idea, in brief, was that while 

 during the Glacial epoch over the entire globe there might be a period 

 of sufficient warmth to produce the desired evaporation, the precipita- 

 tion would fall as rain or snow, according to the local uplift or depres- 

 sion. That the glaciers are now retreating in nearty every instance 

 he regarded as due not so much to a change in climate — at least, not 

 to a gradual increase of temperature — but rather to a gradual decrease 

 in the amount of annual precipitation. He felt that "The weight of 

 the highest authorities is decidely against the theories of both Adhe- 

 mar and Croll, from the standpoint of astronomical science; w r hile it 

 is believed that these theories are equally at variance with the conclu- 

 sions of the most eminent climatologists of the present day. At all 

 events, the evidence in favor of a cyclical recurrence of cold, or 

 Glacial periods, sinks into insignificance when compared with that indi- 

 cating a progressive diminution of temperature on the earth's surface 

 during the geological ages, and from the very earliest time when land 

 began to exist from the conditions of which light on this subject could 

 be procured. 



In this connection it may be mentioned that Whitney considered the 

 movement of glacial ice due to water: 



Glacier ice is not simply ice, but a mixture of ice and water, and it is to the pres- 

 ence of the latter that the whole mass owes its flexibility. The larger the amount of 

 water, other things being equal, the more easily the glacial mass moves. When the 

 water increases so as to get the upper hand, the ice gives way with a rush and 

 becomes an avalanche. * * * The extreme variation of the rate of motion of 

 different glaciers coming down from the inland ice of Greenland is due to the differ- 

 ent amounts of water which they have imbibed. 



The rapid development of the economic resources of Pennsylvania, 

 particularly coal, iron, and petroleum, during the years intervening 

 between the publication of Rogers's final report (1858) and 1874, 

 second Geological aroused a great public demand for more detailed knowl- 

 Penntyivania, ec ^ e °f geological facts. An appeal was, therefore, 



1874-1887. made to the State legislature in 1873 for the establish- 



ment of a second geological survey. This culminated in 1874 in an 

 enactment providing for the appointment of ten commissioners, hav- 

 ing authority to appoint a State geologist "of ability and experience" 

 who should, with ten competent assistants, make such investigations as 

 might be required to elucidate the geology of the State and put the 



« It should be noted that Whitney's ideas were reviewed in a somewhat critical 

 manner by G. K. Gilbert in Science for March 9, 16, and 23, 1883. 



