230 Reviews—Ore Deposits, Alaska Peninsula. 
be heavily charged with debris; Thoroddsen has recently described 
the blocking-up of fiords in Iceland by the debris carried by 
jokull-vatn. 
The glacial theory of the in-and-out channels is also unsatisfactory. 
In the case of a stream flowing between a glacier and a hillside it is 
very probable that the latter would be much more resistant than the 
former, so that the stream would tend to cut its way into the ice and 
to lower the level of its bed. Thus, instead of a flat ‘bench’ along 
the hillside, the latter would be in the form of a shelving slope 
working gradually towards lower ground. Professor Bonney offers 
two alternative explanations: one that the ‘out’ portions are the 
remnants of. valleys, one wall of which has been worn away by 
marine erosion ; the other, that one bank has been greatly steepened 
by warping subsequent to the formation of the channels. 
The widespread occurrence of these dry valleys and the variety in 
form render it probable that no single explanation satisfies all 
the examples. Some must be assumed, at present, to be due to 
‘overflows’, as it is impossible to correlate them with any former 
drainage systems; others, such as the large cross-cut near Cader 
Idris, strongly favour the view that such channels may originate 
without the assistance of glacier lakes. It also seems possible that 
some may arise by the re-excavation or clearing out of older filled-up 
valleys by the action of glacial streams. This would explain why 
these pre-glacial valleys are free of glacial material and at the same 
time satisfy the glacialists who maintain that the form of the trenches 
is that which would be expected from the action of heavily laden 
streams with comparatively low gradients. 
A. 8S. 
VII.—Gerotogy anp Ore Depostrs or Copprr Mountain anp Kasaan 
Prninsora, Atasxa. By C. W. Wricur. U.S. Geol. Sury. Prof. 
Paper No. 87, 1915. pp. 110, with 22 maps and plates. 
({\HE Prince of Wales Island, the largest of the southern islands of 
Alaska, is separated from the mainland by Clarence Strait, 
which with its numerous branches breaks South-Eastern Alaska. into 
numerous islands and peninsulas. The network of fiords extends 
northward from the Portland Canal. The country shows abundant 
signs of glaciation, but has no existing glaciers, and owing to its 
mild moist climate, its latitude of only 55° to 56°, and its long summer 
days, it is covered with a vegetation which, though mainly coniferous, 
is often as dense as tropical jungle. These forests have greatly 
hampered the geological survey of the area, which has been stimulated 
by its valuable copper deposits. The survey by Messrs. F. E. and 
C. W. Wright shows that the country has an Archean foundation, on 
which rest Silurian, Devonian, and Carboniferous sediments. During 
Lower Mesozoic times it was intruded by a great series of plutonic 
rocks, and extensive andesitic lavas were discharged on the surface. 
As usual in Alaska, the chief Kainozoic formation is a great series of 
continental sediments of Eocene age; they were followed by basaltic 
eruptions, some of which have taken place in post-Glacial times. 
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