THE CAUSE OF THE GLACIAL PERIOD. 317 



eral lowering of the temperature ; so that it is doubtful 

 if the astronomical causes introduced by Mr. Croll, even 

 with Dr. Ball's re-enforcement, would produce any appreci- 

 able effect while the distribution of land and water re- 

 mains substantially what it is at the present time. 



Still another variation in the astronomical theory has 

 been set forth and defended by Major- General A. W. 

 Drayson, F. R. A. S., instructor in the Royal Military 

 School at Woolwich, England. He contends that what 

 has been called the precession of the equinoxes, and sup- 

 posed to be " a conical movement of the earth's axis in a 

 circle around a point as a centre, from which it continu- 

 ally decreases its distance," * is really a second rotation of 

 the earth about its centre. As a consequence of this 

 seoond rotation, he endeavours to show that the inclina- 

 tion of the earth's axis varies as much as 12° ; so that, 

 whereas the Arctic and Antarctic Circles and the tropics 

 extend to only about 23° from the poles and the equator, 

 respectively, about thirteen thousand five hundred years 

 ago they extended more than 35° ; thus bringing the 

 frigid zones in both cases 12° nearer the equator than 

 now. This, he contends, would have produced the Gla- 

 cial period at the time now more generally assigned to it 

 by direct geological evidence. 



The difficulty with this theory, even if the mathemati- 

 cal calculations upon which it is based are correct, would 

 be substantially the same as those already urged against 

 that of Mr. Croll. It is specially difficult to see how 

 General Drayson would account for the prolonged tem- 

 perate climate in high northern latitudes during the 

 larger part of the Tertiary epoch. 



It will be best to turn again to the map to observe the 

 possible effect upon the Gulf Stream of a geological event 

 of which we have some definite evidence, and which 



* Untrodden Ground in Astronomy and Geology, p. 26. 

 22 



