600 REVIEWS 
are mostly repeated normal faults, and vary in age from Carboniferous 
(post-Devonian) to Oligocene, with most of the movement probably before 
the Jurassic. 
Novel features of the paper are: (1) The relation of the platform to 
the distribution of gravity (pp. 242-46). A line of deficiency in weight 
has been traced by J. Collet close to the eastern margin of the platform. 
(2) The relation of the structure of the platform to the magnetic phenomena 
(pp..246-49). At the southern edge of the platform, P. Passalsky observed 
coincidence between magnetic anomalies and the position of folds, as well 
as the magnetic character of the rocks. The author ascribes the lack of 
such coincidence at the northern margin to extensive faulting. (3) The 
relation of the platform to earthquakes (pp. 249-52). Earthquakes have 
minimum frequency—about 54; per cent. This low frequency is assigned 
to cessation of deformation within the region, and to the great number 
of repeated boundary faults which protect the platform from foreign shocks. 
C. W. W. 
Geography and Geology of Alaska. By ALFRED H. Brooks, with a 
Section on ‘‘Climate”” by CLEVELAND ABBE, JR., and a topo- 
graphic map and description thereof by R. U. Goopr. (U. S. 
Geological Survey, Professional Paper No. 45.) Pp. 327, 32 
plates. Washington, D. C., 1906. 
The main features of the geography of Alaska are classified and dis- 
cussed in some detail. Four geographic provinces are recognized: (1) 
the Pacific Mountain system; (2) the Central Plateau; (3) the Rocky 
Mountain system, and (4) the Arctic Slope. There are maps showing the 
position of mountain axes, geology, glaciation, and a large topographic 
map on a scale of 4o miles to the inch. The term ‘‘basin lowlands” is 
introduced (p. 281, and Fig. 5) for “ broad, flat depressions, separated from 
the encircling highlands by almost unbroken scarps.”’ ‘‘One of the char- 
acteristic features of this lowland type is that within it the water courses 
are sluggish and aggradation is going on, while the comparatively narrow 
channels of exit are being rapidly cut down.’ All the extensive pene- 
plains of Alaska (Coast Range, Chugach, Yukon, Endicott, and Anaktuvuk) 
are thought to have been formed during post-Kenai (i. e., post-Eocene) 
and pre-Pliocene times. The present great difference in elevation of these 
peneplains is assigned to Pliocene and later deformation. ‘There are a 
few smaller, less extensive base-levels of Pliocene or more recent date. 
C.W: -W. 
