232 Reviens — Sir R. Ball's Ice- age. 



the hemisphere from that of space to that which it actually has, 

 which rise may be perhaps measured by 300° F. ; so that O'l may 

 represent a difference of 30° F. The above may suffice to point out 

 the importance of the work in regard to the astronomical theory of 

 the Glacial epoch. It adds fresh force to Dr. CroU's hypothesis. 



There appears to be a slip at p. 95, where it is said that, " with 

 the present eccentricity of the Earth's orbit, the greatest possible 

 difference between summer and winter would amount to 33 days, etc." 

 Such a difference could only occur when the eccentricity had its 

 highest value of 0-07, whereas at present its value is only about 0017. 



Sir Eobert Ball does not venture to say when the last ice age took 

 place, nor when the next may be expected ; but only that, when 

 they do occur, they will be separated by 21,000 years, with inter- 

 glacial periods intervening. Prof. Darwin, in his notice of this book 

 in " Nature," Jan. 28, regrets this reticence, and inquires whether 

 Leverrier's formu]?e, which Croll used, may not be relied on to givft 

 an approximation to the value of the eccentricity for about 100,000 

 years in the past. Croll constructed an elaborate chart, showing the 

 eccentricity for three millions of years in the past, and one million 

 in the future. Considering the enormous labour with slight mathe- 

 matical powers at his disposal, it makes one sad to think that much 

 of this labour was not more profitable ; but if the formulte can be 

 depended on for 200,000 years in the past, an eccentricity of 0-0569 

 occurred at about that date, which, on his hypothesis, might have 

 been sufficient to bring about the Glacial epoch. ^ 



Looked at from the geologist's point of view, this book seems 

 rather too triumphant. The author appears to think there are fewer 

 difficulties remaining than the geologist would admit. For instance, 

 he attributes the climate of what are now Arctic regions, at the time 

 when a luxuriant flora flourished there, to an interglacial period. 

 But, seeing that a single night of severe frost will kill a fig tree, 

 it is hardly credible that, even with a short winter and a nearer 

 sun, frosts should never have occurred at a place within the Arctic 

 circle, which would have been fatal to such vegetation.- Again, he 

 points out that the astronomical theory necessitates the recurrence 

 of glacial epochs tlu'ough all past geological time, and to explain 

 away the objection that glacial deposits and scratched stones are 

 not to be met with to testify to their frequent recurrence, he refers 

 to the unconsolidated and perishable nature of such deposits. But 

 Till and Boulder-clay are less destructible than many clays, and 

 other unconsolidated deposits, which have been buried again and 

 again under newer strata without being disturbed ; and they occur 

 in India possibly in Pala3ozoic strata.^ 



Other points will occur to the geologist where difficulties appear 

 to be passed over. But although a few passages betray that " The 



^ See a paper on the Ages of the Trail and Warp by the writer, Geol. Mag. 

 Vol. IV. pp. 193-199, 1867, 



^ See discussion on Prof. 0. Heer's paper on fossil plants from North Grinnell 

 Land ; Quart. Journ. Geol. Soc, 1878, vol. 34, p. 70. 



^ Quart, Journ, Geol. Soc, 1878, vol, 34, p. 375. 



