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OC. A. White on the Geology of Southwestern Iowa. 23 
Art. Il.—A Sketch of the Geology o «Speers: Towa; by 
©. A. Wurrs, M.D. 
[In advance of his final report on the Geology of Iowa.] 
WHATEVER may be the theories concerning the natural pro- 
cesses by which magnesian limestone has been formed, present 
ere strongly favor the opinion that with the close of the 
arboniferous period its formation os among the rocks of 
towk Very little limestone has been found among those rocks 
rag designated as the Lower Coal- sriniteds and the only anal- 
es yet made of them are two, reported by Prof. W itney, 
Which yielded him less than one and a half per cent of t 
bonate; but a considerable number of specimens of shoudl here 
referred to t e Upper Coal-measures have been examined by 
Prof. STinrichs the Chemist of the Survey, and in no case have 
they been found to contain so much as one per cent of magnesia 
—usually, only a trace.* A portion of these are perhaps strati- 
graphically equivalent with those in Missouri, which yielded 
pepe one instance upward of twenty-six per cent of the 
car 
The dolomitization of the Devonian and Subcarboniferous 
rocks of Iowa is generally more complete in their northern ex- 
tension, than farther to the sou theatre: In the northern of 
the state the Devonian rocks a proach a true dolomite, but in 
the vicinities of Iowa City and Puveapork although still m 
sian in part, some of the beds are tolerably pure limestone. 
Parts of all the members of the Subcarboniferous series are 
more or less ; the St. Louis limestone which forms the 
upper member of the series in this state, possessing that character 
in a marked degree in some of its beds, both in the northern 
and southern portions. The older rocks len so generally mag- 
nesian that no true limestone has been ized in Iowa 
River shale ing about the same characters as they doin 
the vicinity of esis two hundred miles to the northward. 
* Dr. Geinitz states upon the — of Prof. Marcou that certain of the rocks 
eae 
Analyses of specimens from the same beds 
onl ere miles distant, show only a faint trace of * 
‘4 ogical reports o of Missouri, Dr. Litton gives the results of fourteen 
anal of measure wii i are from the 
middle or lower setien Four of the anal a a 
8°90, one 8-94, and one 26°53. The Averigs of seal the is: two 
and a half per cent of the carbonate. er : 
