112 Review of Hall and Whitney's Geology of Iowa. 
to any distance from the river, and is certainly wanting in north- a 
eastern Iowa, where the Niagara limestone is overlaid directly 
by rocks of the Hamilton group. It appears that a further er 
amination should be made of the section at the Upper Rapids, — 
as it is difficult to understand how so thick a mass of rock should . ; 
appear and disappear without having been recognized anywhere — 
except at that one point. 
tary of the Mississippi, but the economically valuable minerals 
are wanting. This group has not been traced west of 
Mississippi, where, indeed, it exists only in a few detached frag 
n 
ments. 
Of the rocks of Devonian age, the Upper Helderberg Limesion — 
so well marked in New York and Ohio, is with difficulty to be 
recognized "to the west of the Mississippi. Certain non 
iferous strata cropping out on the bank of that river, at, and for 
two or three miles above, Davenport, are referred by Mr. Hall to 
that group, chiefly, as it appears, from their stratigraphical poe — 
tion and lithological character. nee 
The Hamilton group is an important member of the series # 
Iowa, covering many hundred square miles of surface, althoug 
greatly diminished in thickness from what it was in New 10 
It consists of a series of purely calcareous and calcareo-magnesiaa 
strata, with occasional bands in which argillaceous matter 00 of 
to some extent, the lithological character of this porto © — 
the series being somewhat more variable than that of the groups 
below. The greatest thickness of this group exposed in any me : 
section appears to be about one hundred feet; but its entire™™ — 
velopment has not been satisfactorily ascertained. A few © : 
ecies of fossils found in it are identical with those occur! Lael 
ew York; but most of them are new. Several are more *” 
Devonian species of Central Europe than any which had bee 
previously described from this continent. 
