Paleozoic Pocks in the Mississippi Basin. — Keyes. 171 
With the upper portion of the column the difference in the 
determinations of the thickness are even greater than in the 
case of the lower part. In the Coal Measures alone the 
extremes are 600 and 2,000 feet; the first is White’s estimate, 
the latter Broadhead’s. In Iowa the total thickness of the 
Upper Carboniferous was recently placed at 1,600 feet, of 
which the Lower Coal Measures were thought to occupy 
about 400 feet. The result for the latter was based chiefly 
upon deep well drillings. A very careful calculation* was 
also made along a line running from the margin of the coal 
field to the surface outcrop of the base of the upper Coal 
Measures, the exposures being nearly continuous for a distance 
of sixty miles. This and numerous other tests in various 
places shows clearly that the usual method of measuring ex¬ 
posures along a given line and adding them together give 
unreliable results even under the most favorable circum¬ 
stances, and without counting any portion twice. The maxi¬ 
mum thickness of the Coal Measures in the central part of 
the basin is therefore considerably less than even the most 
recent estimates and other data soon to be referred to go 
further to substantiate the statement. 
The coal fields of Missouri and eastern Kansas may be re¬ 
garded as lying on the northern flank of the Ozark uplift. On 
the southern slope, in western Arkansas and the eastern part 
of Indian Territory, the thickness is enormous as compared 
with the more northerly regions, if the estimates recorded are 
correct. ChanceJ ascribes a thickness to the Coal Measures 
of the Choctaw field of 8,500 feet; Winslow,§ to similar strata 
in the Arkansas valley, 10,000 feet; while in the Ouachita 
district Griswold || believes that the columnar section of the 
Coal Measures and Lower Carboniferous have a thickness of 
upwards of four miles, or about 21,000 feet. It is probable 
that all these figures are excessive; yet the strata are un¬ 
doubtedly very thick, as the recent field work by Stevenson^]* 
in the same regions also goes to show. It seems probable that 
*Bull. Geol. Soc. America, vol. n, p. 286,1891. 
■[Missouri Geol. Sur., vol. iv, p. 14, 1894. 
X Trans. American lust. Mining Eng’., vol. xlvii, pp. 653-661, 1892. 
§Bull. Geol. Sur. Soc. America, vol. n, p. 232, 1891. 
Proc. Boston Soc. Nat. Hist., vol. xxvi, p. 476, 1895 . 
^[Communicated, 1895. 
