Review of Hall and Whitney's Geology of Iowa. 111 
Magara Limestone-—This is the third great mass of dolomite, 
which, throughout the valley of the Upper Mississippi, lies next 
above the Hudson river shales, and which, as well from the ex- 
tent of surface covered by it as from its thickness and persisten- 
v of lithological and paleontological characters, forms one of 
the most important members of the Silurian series in the North- 
West. It is one of the rocks which, prior to the recognition of 
the Hudson river shales in the Northwestern mineral region, 
’ ire Limestone.—Of the groups recognized, in New York, 
48 Intermediate between the sing and the base of the Devo- 
nian, but a meagre representation has been observed in Iowa, 
and that only on the Mississippi river. The Le Claire limestone 
" y Mr. Hall asa very heavy bed of dolomite, several 
undred feet in thickness, which, in consequence of its disturbed 
Condition and enduring character, has been the cause of the 
