794 The American Naturalist. [September, 
Period. Time Duration. 
Cenozoic, including Pleistocene  . i à . 2,900,000 years. 
Mesozoic, : ; : : : : : 7,240,000 = 
Paleozoic, . x i i > : : e 110500000 .* 
Algonkian, ‘ A : : i 4 : 17,500,000 “ 
Archean, . ‘ ‘ : i : i . 10,000,000(?) “ 
* [tis easy to vary these results by assuming different values for area 
and rate of denudation, the rate of deposition of carbonate of lime, 
etc. ; but there remains, after each attempt I have made that was based 
on any reliable facts of thickness, extent and character of strata, a 
result that does not pass below 25,000,000 to 30,000,000 as a minimim 
and 60,000,000 to 70,000,000 as a maximum for post-Archean geologie 
time. I have not referred to the rate of development of life, as that is 
virtually controlled by conditions of environment." 
* [n conclusion, geologic time is of great but not of indefinite duration. 
I believe that it can be measured by tens of millions, but not by single 
millions or hundreds of millions of years" (Journ. Geol, Vol. I, 
1893.) 
For the latest estimates as to the duration of the Glacial period see 
AMERICAN NATURALIST, March, 1894, p. 263. 
The Lignites of Southern Chili.—After having made a field 
study of the lignitic formation in the southern part of Chili, M. Noguès 
reports to the Société Scientifique of Chili that these lignites certainly 
do not belong to the Permo-carboniferous age, as has been stated, but 
are of a much later age. They constitute a long band extending in a 
north and south direction, parallel with the Pacific Ocean, and have 
been dislocated by a complex series of faults. M. Noguès extended 
his observations to the schisto-arenaceous system, which is found around 
the river Bio-Bio and its affluents, La Quilacoya and the Rio Grande, 
and which contains beds of true anthracite coal. Paleontological evi- 
dence shows that this system corresponds with the lower beds of the 
lignitic formation above mentioned. Like the lignite, also, it rests 
unconformably upon granite rocks and the old schists of the Cordil- 
leras, and been subjected to movements which have produced folds, 
swellings and anticlinals. (Actes de la Soc. Sci. du Chili, Santiago, 
1894.) 
Lower Cretaceous Fossils from the Black Hills of 
Dakota.—A recent]find of cycadean trunks near Hot Springs, South 
Dakota, led Mr. Lester Ward to investigate that locality with the view 
