Eevieivs — The Geology of Illinois. 473 



Prof. J. D. Whitney's "driftless region;" ^ but Mr. Worthen thinks 

 he has found many transported pebbles within it, showing that the 

 ice must have passed over it. The iceberg theory is adopted to 

 explain the phenomena. M. Lesquereux presents in full his 

 theory of the origin of prairies. He thinks they were originally 

 bays or shallow portions of fresh-water lakes, which gradually 

 changed into swamps with an abundant aquatic vegetation, and 

 finally, by drainage, became considerably elevated, and from the 

 plants, mullusks, and diatoms, a rich soil developed itself. The 

 high rolling prairie results from the occurrence of a large number 

 of swamps partially separated by sandy banks, and often comes 

 from simple denudation. He thinks the aquatic origin is shown by 

 the natural absence of trees. 



The Carboniferous system in Illinois attains a maximum thickness 

 of 2500 feet, and underlies three-fourths of the area of the State. 

 The statements in respect to the number and succession of the 

 Coal-beds and contiguous strata contained in the first two volumes 

 are corrected in the first chapter of the third volume. It appears 

 that the generalizations of M. Lesquereux^ and others concerning 

 the equivalency of Coal-beds in Pennsylvania, Kentucky, etc., are 

 based upon an erroneous stratigraphy. The "Mahoning Sandstone" 

 and "Anvil Eock " were supposed to represent horizons 350 feet 

 apart, including eight beds of coal. But a careful examination by 

 Messrs. Worthen and Lesquereux along the Illinois Eiver for one 

 hundred miles, in 1867, proves these two arenaceous bands to be 

 identical with each other ; and hence the beds of coal, at first 

 referred to an age posterior to the Mahoning Sandstone, are now 

 seen to be anterior to the epoch of its deposition. This change in 

 the stratigraphical column must necessitate some modifications in 

 the inferences formerly derived from it. 



As now revised, the Illinois section shows ten beds of coal in a 

 vertical thickness of 600 feet in the central and northern parts of 

 the State. Six of these average from two and a half to six feet in 

 thickness, and the others range from a few inches to two feet. The 

 thickest beds are in the lower division of the measures. In addition 

 to the ten seams just mentioned, Prof. Worthen speaks of several 

 " local beds " of coal in the Millstone Grit. Permo-Carboniferous 

 fossils are found in the upper part of the Coal-Measures of Illinois, 

 showing that the whole of the series is present. 



M. Lesquereux's position in respect to the identification of Coal- 

 Measures is this : Beds of coal can be identified in different basins 

 by the peculiar assemblages of plants found in connexion with them. 

 For example. No. 2 of Illinois is characterized by the presence of 

 leaves of Lepidodendron, and scarcely anything else. Its supposed 

 equivalent in Pennsylvania, the "Mammoth Bed," is largely the 

 same in character, carrying also the class of fruits represented by 



1 Geology of Wisconsin, by J. Hall and J. D. Whitney, 1862, p. 114. 



2 Geological Eeport of Kentucky (Owen), vol. iv., p, 331. Amer. Journ. Sci., it. 

 vol. XXX., p. 367. 



