198 REVIEWS—GEOLOGICAL SURVEY OF IOWA. 
Amongst the more interesting facts discovered by Professor Halls’ 
exploration of Iowa, may be cited, first of all, the occurrence of the 
Hudson River Group in that State and in Illinois; represented by 
more or less bituminous shaly layers, having an entire thickness of no 
more than sixty or seventy feet, and thus affording a remarkable exem- 
plification of the law of decrease of sedimentary matter in the west- 
ward extension of the paleozoic beds. These Hudson River shales 
are the equivalents of the “ Blue Limestone’’ of Cincinnati (former- 
ly thought to be Trenton), and the “Blue Shale’? of Wisconsin. 
They appear to be principally exposed around Dubuque, or in a 
narrow band from the vicinity of Bellevue to somewhere about the 
head waters of the western branches of Turkey River; growing 
gradually thinner and thinner, until they finally die out. Another 
fact of no little geological interest, brought out by this survey, is the 
discovery of a magnesian limestone formation lying above the Niagara 
Limestone, and not previously recognised in the Mississippi valley. 
Professor Hall has named this the Leclaire Limestone, from its 
development around that locality. It occupies, apparently, a con- 
siderable area, having within its limits the main portion of the Wap- 
sipinecon river; and it forms more especially by its undulations the 
so-called “ Upper Rapids” of the Mississippi. Professor Hall shows 
that these Leclaire beds occupy the same geological horizon as the 
Galt limestone of Canada West, and he is inclined to look upon the 
two as geologically identical. _ If this, on further examination, prove 
to be the case, it will furnish an additional argument for the separa- 
tion of the Galt beds from the Onondaga salt group with which at 
present they are conventionally placed. 
In his very interesting sketch of the physical geography of Lowa, 
Mr. Whitney enters on a somewhat extended description of the 
causes to which the absence of arboreal vegetation on the prairie lands 
may be considered due. He attributes the principal cause of this, 
and apparently on conclusive grounds, to the peculiar and highly 
comminuted condition of the surface soil. “‘ Taking into considera- 
tion all the circumstances under which the peculiar vegetation of the 
prairie occurs, we are disposed to consider (he remarks,) the nature 
of the soil as the prime cause of the absence of forests and the 
predominance of the grasses over this widely extended region. And 
although chemical composition may not be without influence in 
bringing about this result, which is a question worthy of careful 
