312 Tlw American Geologist. Xovember, i89» 
each recurrence. Moreover, it offered so tempting a means 
of linking- together geological and astronomical times that it 
awakened a hope that the two might ere long by its agency 
he united. The fundamental fact also — the secular elongation 
of the major axis of the earth's orbit — was so firmly estab- 
lished that it admitted no dispute, and the secondary principles 
deduced from it were invested by the skill and eloquence of 
the author with such a halo of efficiency and probability that 
no one need feel any surprise at the rapid advance of the 
theory in the geological world. 
But with the progress of research there have "cropped 
out" certain difftculties and objections which were but slight- 
ly, if at all, conspicuous at the outset. Among these is the 
condition demanded by Dr. Croll's theory that the arctic and 
antarctic eras of glaciation were not contemporaneous but 
alternate. With this it stands or falls. But geologists can 
find no evidence for the alternation. On the contrary all the 
facts bearing on the question, though few, fail to show any 
difference in date. The ice-marks, whether scratches, 
grooves, or boulders, are equally fresh in both hemispheres, so 
that it is impossible to believe in a difference of 20,000 years 
in their age as required by the theory. Then the work done 
on the surface since the ice passed away is too little to fill the 
long interval of 80,000 years which Dr. Croll assigns to the 
past glacial era. The gorges of Niagara and of St. Anthony 
are too small to have supplied occupation for their streams 
during so long a time. Many other similar facts point very 
strongly in the same direction, so that geologists generally 
are more disposed to bring the close of the cold era and the 
disappearance of the ice within ten or fifteen thousand years 
than to admit the 80,000 years of the Croll hypothesis. Again, 
the apparent irregularity and want of contemporaneity shown 
by the glacial phenomena in various places indicate a less 
general cause than one whose action extended at once over a 
half of the globe. 
These and other considerations have during the past few 
years gradually undermined the hypothesis of the brilliant 
Scotch geologist, so that it may now be said to stand much 
less securely than it did twenty years ago. In spite of its ac- 
counting for the recurrence of glacial and interglacial condi- 
