196 REVIEWS—GEOLOGICAL SURVEY OF IOWA. 
by J. D. Whitney, and sundry geological details by A. H. Worthen 
—has been issued within the last few months by the Legislature of 
Towa. The paleontological portion of the Report is bound up 
separately. It contains some twenty-nine or thirty steel-plate en- 
gravings of very superior execution, exhibiting about a thousand 
figures of the more characteristic or remarkable fossils collected 
during the prosecution of the Survey. Mere sketches of scenery, 
on the other hand, however pleasing in themselves, have been very 
properly dispensed with in this Report. Illustrations of that kind 
add enormously to the costs of publication, without offering, as a 
general rule, any compensating advantages. 
The oldest recognised rock in the State of Iowa appears to be the 
Potsdam Sandstone, This, blended intimately with the Caleiferous 
Sand Rock, is sparingly developed along the line of the Mississippi, 
in the extreme North-East corner of the State. The other subdivi- 
sions of the Silurian Series, and those of succeeding formations up 
to the coal measures, follow in more or less regular gradation, with 
their lines of strike running in a general N.W. and 8 E. direction, 
or, as stated by Professor Hall, at right angles to the Cincinnati axis 
and the lines of disturbanee along the Appalachian Chain. Owing 
to this direction of the strata, they are cut successively by the 
Mississippi River, and show from north-east to south-west the follow- 
ing sequence :—The Potsdam Sandstone and Calciferous Sand Rock ; 
the St. Peters Sandstone; the Trenton beds; the Galena limestone 
(locked upon as an upper portion of the Trenton Group ;) the Hud- 
son River Shales, showing only a narrow outerop-band; the 
Leclaire Limestone (see below); the Onondaga Salt-Group, the 
equivalents of the Upper Helderberg Limestone, Hamilton, and 
Chemung groups; the Carboniferous Limestoue; and the Coal 
Measures. Although these follow one another regularly, here and 
there an underlying division is exposed by denudation or river- 
eutting in some of the tributary vallies of the Mississippi. Thus, 
amongst other examples, the Trenton Limestone re-appears within 
the Galena Limestone area along the line of Turkey River; and , 
the Carboniferous Limestone, within the area of the Coal Mea- 
sures, along the valley of the River Des Moines. Professor 
Hall reinarks, that, in tracing westward such of these geological 
formations as are known in New York and Pennsylvania, they 
are found to thin out gradually, becoming indeed, in some in- 
