384 The American Geologist. "^^"^' ^^"^■ 
forty or fifty miles southward, lignite beds fifteen feet thick are not un- 
common, and some extensive seams are twenty-five feet thick. 
During the year 1902 the output of North Dakota lignite coal was 
296,800 tons, with a value, at the mines, of $407,000, being more than 
twice the amount and value of its production in the year 1900. 
Professor Babcock contributes as the final part of this report, an im- 
portant paper of forty-three pages, on- the "Water Resources of the 
Devil's Lake Region." It is found that this lake during the last twenty 
years has ranged from four to nine feet lower than it was in 1883 ; and 
its ancient high shore lines are twenty to thirty feet above the present 
water level. The greatest depth of water now found by soundings, in 
the central part of the main lake, is twenty-nine feet. 
Thus the alkaline Devil's lake, so called in translation of its Sioux 
name, probably alluding to the bitterness of its water, which is the 
largest lake of North Dakota, about thirty-two miles long, with a very 
irregular area, has now no more than a sixth or an eighth so much 
volume as immediately after the retreat of the ice-sheet from this area, 
when, as an enlarged expanse of fresh water, it reached east into the 
basin of Stump lake, and thence outflowed southward to the Sheyenne 
river. w. u. 
The Cause of the Glacial Period, being a Resume and Discussion of the 
Current Theories to account for the Phenomena of the Drift, zvith 
a Nezu Theory by the Author. By H. L. True, M.D., Member of 
the Ohio State Academy of Science. Pages xi, 162 ; with 7 plates, 
and 9 figures in the text. Cincinnati. The Robert Clarke Co., De- 
cember, 1902. 
The various theories which attribute continental glaciation to chang- 
es in the astronomic relations of the earth, as advocated by Croll, Ball, 
Drayson, Becker, and others, are reviewed and discarded by Dr. True ; 
and the usual theories appealing to geographic changes, as great ele- 
vation of the lands that became enveloped by snow and ice, held by 
Lyell, Dana, LeConte, Upham, Very, Wallace, Wright, and others, are 
also considered insufficient or improbable. 
Variations in the amount of heat received from the sun, or in its 
retention by the earth's atmosphere, with changes in the proportions of 
water vapor and of carbon dioxide in the air, as recently proposed by 
Arrhenius and Chamberlin to account for the Glacial period, are also 
thought to be inapplicable to this problem. , 
Instead of all these rejected hypotheses. Dr. True advances one 
original with himself; which, however, agrees with the view presented 
by Upham, in an appendix of Wright's "Ice Age in North America," 
by associating the glaciation of continental areas with the very extra- 
ordinary crumpling of the earth's crust and upheavals of mountain 
ranges which have characterized Late Tertiary and Quaternary time. 
The new theory is stated as follows : "Up to and during the princi- 
pal part of the Tertiary period the earth had so far cooled and the 
rocky crust had become so thickened that it sustained the pressure of 
