Review of Recent Geological Literature. 119 
at some level horizon far below the surface the weight of the overlying 
matter is everywhere the same. The first is called the doctrine of 
terrestrial rigidity, and the second the doctrine of isostasy. If we ac- 
cept isostasy, the second problem is expressed in the question whether 
the difference between the continents and ocean floors in their density 
is due to a difference in their temperature or arises from differences in 
composition. Proceeding next to the question of the origin of the 
continents, the only or chief hypothesis which has been propounded, 
namely, that of Dana, is regarded as perhaps accordant with the theory 
of isostasy, but in need of more full comparison with the increasing data 
of recent years. The author's studies of the moon led him to suspect 
that the continents may have begun to exist on account of accessions to 
the matter of the earth's equatorial tract ; but this hypothesis is set 
aside because the great circle most coincident with the continental areas 
passes through the poles. Differential elevation and subsidence of the 
continental plateaus are believed by Mr. Gilbert to depend on causes 
which are yet unknown and have probably not yet been suggested. 
Again, the general permanence of the continents, though highly prob- 
able, is not yet fully demonstrated; and the doctrine of the growth of 
continents, although generally accepted, is shown to be still open for 
profitable discussion. 
Comparison of Pleistocene and Present Ice-sheets. By Warren 
Upham. Bulletin G. S. A., vol. iv, pp. 191-204; March 24, 1893. From 
the ascertained slopes of the surface of the Greenland ice-sheet, the 
author infers that the glaciated areas of North America and Europe 
were much uplifted, so that the borders of their Pleistocene ice-sheets 
had similarly steep slopes, the amount of uplift for the central portion 
of Canada being thought to be not less that 5,000 feet. From the rate 
of erosion of the Muir glacier, found by Wright and Reed to average 
about three-fourths of an inch yearly, it appears that a period of per- 
haps 10,000 or 20,000 years would suffice for the observed volume of the 
Pleistocene glacial erosion and resulting drift. The recent exploration 
of the Malaspina glacier or ice-sheet in Alaska by Russell is thought to 
sustain the view that the ice-sheets of the Glacial period had much en- 
glacial drift; that during their departure the annual amount of ablation 
may have been twenty-five feet or more, so that near the glacial bound- 
ary a thickness of 2.000 to 4,000 feet of ice would be melted in one or 
two centuries; that large portions of the englacial drift were car- 
ried away by streams to form eskers, kames and valley drift; and 
that forest beds enclosed between deposits of till probably in most or 
all cases were formed by only moderate re-advances of the ice-sheets. 
The chief evidence which has been supposed to prove interglacial epochs 
appears therefore consistent instead with the view that the Ice age was 
continuous and geologically brief. 
Cretaceous and Earl;/ Tertiary of northern California and Oregon. 
By J. S. Dili.er. Bulletin G. S. A., vol. iv, pp. 2(C) 224, with maps; 
April 14, 1893. The Wallala formation of Becker and White is shown, 
through Dr. DalTs observations, to be a phase of the Chico beds, which, 
