174 The American Geologist. ' ^^'■<=^^' i^^^- 
EDITORIAL COMMENT. 
CROLL'S THEORY REDIVIVUS. 
Glacialists will read with much interest the discussion of 
the climate of Mars by Mr. Percival Lowell, published in the 
Proceedings of the American Philosophical Society,* Phila- 
delphia. 
Twenty-five years ago, when Croll's theory was tested by 
an appeal to the testimony of Mars on the bearing of eccentri- 
city on glaciation, it was because Mars presented appearances 
suggesting polar ice-caps, and because he is in eccentricity and 
tilt favorable to the application of Croll's theory. It was im- 
mediately discovered that Mars, on the assumption that other 
conditions were similar to those of the earth, afforded no sup- 
port to that hypothesis. One pole showed as large a cap as the 
other. Eccentricity therefore did not seem to affect precipi- 
tation about the pole. 
But since then the examination of the physical conditions 
of Mars has proceeded. It has been discovered that the as- 
sumption of similarity with the earth was a mistake, and that 
Mars presents the opposite of similarity with the earth, "and 
with the flight of the similar the cogency of the argument de- 
* parts." In the light of this change Mr. Lowell re-opens the 
case and proceeds to discuss anew the bearing of the planet 
Mars on the hypothesis of Croll. 
Accepting the suggestion of Sir William Herschel, that the 
white spots that appear on Mars about his poles, are due to ac- 
cumulations of ice and snow, and noting that they increase al- 
ternatingly and diminish again "in a certain chronometric ca- 
dence," he confronts at once the theory that those spots are due 
rather to congealed carbonic acid, for carbonic acid in extreme 
cold not only assmues a solid form, but that fonn is as white 
and delicate as snow. He calls attention to the reluctance of 
carbonic acid to remain in a liquid form. It passes almost im- 
mediately from a solid to a gas. There are, however, certain 
features that appear about the margins of those white caps 
that show rather plainly that they disintegrate as they shrink, 
giving rise to belts and bays of different color. These mar- 
ginal features behave precisely as if they were affected by the 
• Op. cit., vol. 39, Oct.-Dec, 1900. 
