Biblioyiaiphical Notices 167 
- The Coal-measures of the State are geologically continuous with 
those of Illinois and Wisconsin. They are unconformable to the 
older Palzeozoic rocks, and overlap the edges of the slightly inclined 
strata of Devonian and Silurian age. As yet, in lowa the Coal- 
measures have been proved to the depth of 500 feet only; whilst 
in Missouri they are 640 feet thick, in the Mississippi Valley 1000 
feet, in Pennsylvania upwards of 6000 feet (and, including the Con-. 
glomerate, nearly 8000 feet), in Cape Breton more than 10,000 feet, 
and in Nova Scotia upwards of 14,000 feet. Only one or two seams 
of coal (from 3 to 5 feet or more thick) and a cannel-coal are worked 
in Eastern Iowa. 
In Western Iowa the Permian formation is said to be represented 
by gypsiferous rocks; and the Lower Cretaceous rocks are well- 
developed both there and in the neighbouring Territories of Kansas 
and Nebraska. Mr. Hall, from the evidences already collected in 
the West, is inclined to believe in the existence of an extensive de- 
velopment of Jurassic rocks along the eastern side of the Rocky 
Mountains. ; 
- It would appear,” observes the author, ‘ that, from the earliest 
Silurian times, the Great West, or the region of the Rocky Moun- 
tains, has been an ocean which successively received the finer sedi. 
ments derived from eastern lands, or which produced within its own 
area the caleareous deposits,—but ever an ocean, not only to the 
close of the Carboniferous period, but still later through the Per- 
miab, Jurassic, and Cretaceous periods ; showing apparently no evi- 
dences of dry land till about the beginning of the Cretaceous era, or 
perhaps a little earlier ; while, in later Tertiary periods, the conti- 
nental fauna and flora have been remarkably developed over the same 
area. Thus, while the older Paleeozoic formations have been: largely 
- accumulated in the East, in successive beds, having altogether a 
thickness of several times the height of our highest mountains (in 
Towa), they have greatly diminished in the West. At the same time, 
while the post-palzeozoic formations are very thin or often absent in 
the East, they have accumulated in vast amount along the line of 
the Rocky Mountains, from one end of the continent to the other.” 
‘p. 141). 
rhe iyeioal geography of the country is described by J. D. Whit- 
ney, in chapter 1; the general geology, by J. Hall, in chapters 2 
and 3;. the geology of the Desmoins Valley, by A. H. Worthen, in 
chap. 4; the geology of certain counties, by A. H. Worthen and 
J. D. Whitney, in chaps. 5 and 6; and the chemistry and economic 
geology by J.D. Whitney, in chap. 7, It is stated that “the geo- 
logical structure of the State clearly indicates that her agricultural 
products, her coal-mines and beds of gypsum will constitute her 
greatness of resources and her future wealth” (p. 145); and the 
lead-mines are also of importance. 
_ Chapter 8 of this work (or part 2 of vol. i.) treats of the paleeonto- 
logy of the State as far as concerns the following groups of rocks,— 
Hamilton group, Chemung group, Burlington Limestone, Keokuk 
