MORAINES OF RECESSION 427 
the chief causal condition. But whatever the cause of the Ice 
age itself may have been, it seems hardly possible to account for 
the moraines of recession by any scheme of ups and downs of 
the solid earth. The moraines themselves indicate that the 
oscillations of the ice-front were of a periodic nature, and there- 
fore dependent upon the operation of a periodic cause. All 
geological forces that are purely terrestrial are necessarily derived 
from the interior of the earth, and there is no evidence that their 
activities are periodic, although they may recur at irregular 
intervals. Still less is it possible to conceive of true periodicity 
in the surface manifestations of purely terrestrial forces, such, for 
instance, as would be required to explain periodic movements of 
elevation and subsidence over wide areas, especially where the 
amount of the successive movements would have to be regulated 
to the extreme nicety of progressive variation requisite to pro- 
duce the climatic cause of the moraines of recession. All such 
supposable terrestrial causes may therefore be safely put aside. 
To find a source for periodic causes we are compelled to turn 
to astronomy. According to established doctrines the only way 
in which astronomical forces can be supposed to influence glaci- 
ation is through climate. The annual period of climatic change 
is so short that it is, of course, out of the question. A period 
of climatic change in rounds of about thirty-five years has been 
deduced by Forel and others from the study of the variations of 
glaciers and of rainfall.* But, as will be shown farther on, this 
too seems far too short. After this the only known period of 
climatic variation is that due to the precession of the equinoxes, 
1F.-A. FOREL. (Archives. Sci. Phys. Nat., May 15, 1886, p. 503.) Fore] points 
out that the variations of rainfall and air temperature, as deduced by C. Lang, agree 
with his own periods of variation in Alpine glaciers. (Also Am. Jour. Sci., for July 
1886, p. 77.) In Forel’s latest writing on this subject (“Les Variations Periodiques 
des Glaciers,” Genéve, 1895) he finds the grounds for deducing a definite period to be 
rather unsatisfactory. He finds that glaciers of different sizes and lengths do not show 
the effects of causes of advance or retreat synchronously. After a few seasons of 
increased precipitation all glaciers tend to advance, but small ones advance sooner 
than great ones so that they do not attain their maxima at the same time. A small 
glacier will reach its maximum and get far back on its retreat before a greater glacier 
attains its maximum advance —a result that is natural from the fact that the effects 
