276 _ The American Naturalist. [March, 
ancient volcano which rose out of and deposited its débris in the deep 
water of the Upper Cretaceous sea (probably Niobrara sub-epoch), 
From its isolated position, remote from any contemporaneous shore- 
line, it must have been an island eruption. Pilot Knob probably 
belongs to a great chain of igneous localities, eruptive and basaltic, 
extending from the mountains of Northern Mexico to the Ouachita 
system of Arkansas, both of which regions abound in related features, 
The great Balcones system of N. 20° E. faults of Central Texas are 
later than Upper Cretaceous. In late Cretaceous and Tertiary times 
Pilot Knob was either totally submerged or greatly denuded.” (Am. 
Geol., Nov., 1890. $ 
The Sierra Nevada of Central California.—During the past 
season G. F. Becker has studied the structure of the Sierra Nevada _ 
Mts. in the neighborhood of the Stanislaus and Truckee Rivers, with 
the following results : i 
The whole area in this region has been glaciated up to the sum- 
mits of passes. There are six systems of fissures. The fissures are 
fault planes. The disturbances which caused the fissures happened 
since the close of the Miocene. The faults rarely exceed three inches, 
A careful study of the vertical fissures leads to the hypothesis of a 
horizontal thrust acting on a south-southwest to north-northeast line. 
r. Becker advances arguments to show that no important tilting of 
this portion of the Sierra has taken place at or since the post-Miocene 
disturbances. The paper closes with the assertion that the theory that 
the earth is a solid highly viscous mass, is in all respects compatible 
with the observations, fully explaining every one of the six fissure 
systems, the faults observed, and the enormous resistance to tilting 
which the range has displayed. (Bull. Geol. Soc. Am., Vol. IL., pp- 
49-74). 
The Origin of the Great Lakes.—In discussing the origin of 
the basins of the great lakes of America, J. W. Spencer concludes 
that the valleys of Lakes Erie, Huron, and Michigan are the result of 
erosion of the land surfaces by the ancient St. Lawrence River and its 
tributaries during a long period of. continental elevation, and that 
meteoric agencies had broadened the valleys. This condition was at 
its maximum just before the Plistocene period. The closing of portions 
of the old Laurentian valley into water-basins occurred during and 
at the close of the Plistocene period, owing, in part, to Drift filling 
some portions of the original valley, but more especially to different 
warpings of the earth’s crust. (Quart. Geol, Soc., Nov., 1890.) 
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