STRATIGRAPHICAL GEOLOGY 
255 
life-zones, as well as others less conspicuous, pass without slight¬ 
est interruption from the unaltered limestone to the dolomite. 
These relations are best indicated by diagram as represented be¬ 
low (Fig. 18). 
Hall was not, as is commonly supposed, the first worker to 
determine the faunal position of the Ordovicic section in Iowa. 
When the author of the Galena Limestone arrived on the ground 
in 1855, he found that the Blue unaltered lime rock of the Du¬ 
buque region had, by J. N. Nicollet already been paralleled, 
on the basis of its contained fossils, with the Trenton group of 
New York. Likewise D. D. Owen had recognized in the Iowa 
rocks organic remains which he regarded as characterizing the 
Trenton horizon of the East. 
Late Cretacic Formations in English Channel. The existence 
of Late Cretacic rocks in the central deep of the English Chan¬ 
nel was recently ascertained from the results of the dredgingsi of 
the “Pourquoi Pas?’^ 
Flint nodules, entirely analogus to the flint nodules of the 
Chalk of the Paris Basin, cover in very great quantity the whole 
bottom of the central depression of the English Channel. The 
10 Rept, Intended to Illustrate Map of Hydrographic Basin of Upper 'Mississippi 
River; Sen. Doc., 26 Cong., 2nd Sess., Vol. V, pt. ii. No. 237, Washington, 1841. 
11 Rept. Geol. Surv. Wisconsin, Iowa and Minnesota, 638 pp., Philadelphia, 1852'. 
