Present Problems of Geophysics. — Becker. 1 5 
period seems generally accepted. CroU's theory is discred- 
ited. I have shown to my satisfaction that the astronomical 
•conditions most favorable to glaciation are high obliquity 
and low eccentricity of the earth's orbit,t but can not claim 
any extensive following. If I am right, it should be possible 
to obtain a definite measure of geological time in years as 
soon as the astronomers have completed the theory of secu- 
lar variations in the planetary system so far as to be able to 
assign the lapse of time betv/een successive recurrences of 
low eccentricity and high obliquity. 
A most interesting observation, which promises much 
light on the past history of the globe, is that lavas and strata 
indurated by lavas retain the polarity characteristic of the 
locality in which they cooled.? The time may come when this 
will lead to determinations of the relative age of lavas, the 
duration of periods of eruption and possibly even absolute 
determinations of date. 
Geology has long, and with some justice, labored under 
the reproach of inexactitude. As has been illustrated in the 
preceding pages, the science is still in the qualitative stage 
and almost wholly lacks the precision of astronomy. Even 
its most ardent students have seldom succeeded in ascertain- 
ing the quantitative relations between efifects and operative 
causes and have been perforce content to indicate tendencies. 
Thus geological doctrine is far too much a matter of opinion, 
"but this is hardly the fault of the areal geologist. The coun- 
try must be mapped both for economic reasons and to accu- 
mulate a knowledge of the facts to be explained. Working 
"hypotheses the field geologist must have, or lie could not 
prepare his map ; and he is only responsible for Hving up to 
the standard of knowledge of his time. He is continually 
face to face with phenomena for which physics and chemistry 
should account, though they may have not yet done so. and 
must accept seeming probabilities were certainty is unattain- 
able. So, too, Kepler's predecessors recorded facts and 
■guessed at generalizations as best they might. 
The physics of extreme conditions still awaits satisfac- 
tory exploration. The geologist turns to the physicist for 
t Amer. Jour. Set., vol. xliii, 1894, p. 95. 
J Bbunhks and David. Comptes Rendus. vol. cxxxiii, 1901, p. 153. 
