CAUSES OF GLACIAL PERIODS 41 



the direction of the great warm currents of the ocean, 

 together with changes in the growth of forest and the 

 extent of desert land on the continents, might, by affecting 

 the habitual disposition of cloud and of vapour, go far to 

 reduce the average annual temperature of north temperate 

 regions by 10°. 



It has, on the other hand, been held that the periodic 

 and regular " wobbling " of the earth as it spins on its 

 axis is largely accountable for the fall of temperature 

 in the temperate zone at regularly recurring intervals. 

 The axis of rotation of the earth moves round in a circle, 

 as one may see the stem of a well-spun top slowly 

 move in a circle whilst the top " sleeps." The earth takes 

 about 26,000 years to complete its wobble, and in that 

 cycle there is a period when there is least and a period 

 when there is most sunshine falling on the polar regions — 

 owing to the difference in the inclination of the pole to the 

 sun. It is held that this difference is enough to produce 

 the fall of 1 0° F. required to give us a " glacial period " in 

 this part of the world. At any rate, in combination with 

 the changes conducing to formation of vapour and cloud 

 which I have mentioned above, it would probably be 

 effective. 



Geologists are not agreed on this subject, but they have 

 established, as I have stated above — by definite proof — the 

 recurrence of glacial periods separated by long intervals of 

 warmer climate during the latest period of geological 

 time, the Pleistocene. The most convincing proof of the 

 occurrence of three periods of great extension of European 

 glaciers with intervals of a milder climate has been 

 obtained by studying the ancient moraines. The great 

 mass of heaped-up rock fragments left as a morame by a 

 once extended glacier which has dwindled and retreated, 

 becomes altered on its surface in the course of a few 

 thousand years by change and decomposition of the rock 



