REVIEWS 603 
Geology oj the Lower Colorado River. By Wiius T. LEE. (Bulletin 
of the Geological Society of America, Vol. XVII [June 23, 1906]. 
Pp. 275-84, plates 32-24.) 
In this interesting paper the origin of the Grand Canyon is placed at 
the very close of the Tertiary and most of its erosion is assigned to the 
early Quaternary. During the Pliocene and early Quaternary the Lower 
Colorado flowed through a broad valley occupied by Virgin River at the 
north and extending southward 125 miles to the mouth of Williams river. 
It was deflected from this course by detrital aggradation and lava flows 
which filled a part of the old valley to a depth of 800 feet. Since then 
it has lowered its new channel over 2,000 feet and cut Boulder, Black, 
Needles, and Aubrey canyons. The canyon has since been. filled with 
gravel to a depth of 700 feet, and later re-excavated below its present 
bottom. At present aggradation is in progress. Ewe Wi: 
Geology and Mineral Resources oj Part of the Cumberland Gap Coal 
Field, Kentucky. By GroRGE HALL ASHLEY AND LEONIDAS 
CHALMERS GLENN. (U. S. Geological Survey, Professional 
Paper No. 49.) Pp. 239, 40 plates. Washington, D. C., 1906. 
The rocks of this field all belong to the Pottsville group, with a thick- 
ness here about 4,000 feet. The lower third of the rocks are mainly sand- 
stones, while the upper two-thirds, carrying the coal beds, are about equally 
sandstone and shale. ‘The structure is that of a flat-bottomed U-shaped 
trough; dips do not average more than roo feet to the mile. There are 
thirteen workable coals. Eight are mined; thickness, 4 to 6 feet. The 
output is from 600,000 to 1,000,000 tons a year, used mainly by the L. & 
N., and the Southern Railways and by a blast furnace at Middlesboro. 
C. W. W. 
Trent River System and Saint Lawrence Outlet. By ALFRED W. G. 
Witson. Rochester, N. Y., 1904. (Bulletin of the Geological 
Society of America, Vol. XV, pp. 211-42, Plates 5-10.) 
The configuration of the Trent River system of eastern Ontario is 
determined by joints in the Ordovician limestone of that area. Farther 
north in the Archean, pre-Ordovician faults influence the drainage. Neither 
glacial erosion nor deposition have been sufficient to obliterate the pre- 
glacial drainage lineaments determined by joints. But the pre-glacial 
direction of drainage was prevailingly to the southwest and this has been 
largely changed by tilting. 
