HISTORICAL NOTICES. 21 



icy pall over the geology of the later tertiary periods, is 

 fast melting away before the sunshine of truth." 



Perhaps I was a little too sanguine as to the rapidity 

 of the process, and did not make allowance for that 

 chilling current of popular text -books and official 

 influence which has so much retarded the final melting of 

 the great continental glacier and the polar ice cap. 



The following citations, however, from very recent 

 publications show that my forecast of the course of opinion 

 was not altogether wrong, and that we may hope for still 

 better things in the future. 



Sir Eobert Ball's recent attempt to rehabilitate Croll's 

 ingenious astronomical theory of the glacial age will not 

 assist to restore its waning fortunes, but brings out the 

 fact that this ingenious theory was essentially defective.* 

 He shows that Croll reasoned on a mistaken assumption 

 that the earth receives equal amounts of heat when in 

 perihelion and aphelion passage, whereas the difference is 

 as much as 26 per cent., and consequently at long 

 intervals there might occur periods of great coldness in 

 one hemisphere at a time. The interval of time, however, 

 is too long, even on Ball's theory, and the fact that the 

 ice of the glacial period radiated from points consider- 

 ably south of the polar circle, tells of the dominancy of 

 terrestial conditions. This new astronomical theory will 

 therefore fail to affect geological conclusions, and its weak 

 points have already been pointed out by geological 

 reasoners. It may, in short, be held to have given a 

 death blow to the theory of astronomical causes of the 

 glacial period known to us in geology. 



In a recent paper, Mr. Warren Upham, one of the most 



* " The Cause of an Ice Age," London, 1892. 



