426 Reviews — Geology of Illinois. 



Survey of the State. The work was commenced, but until after Mr. 

 Worthen's appointment in 1858 no reports appear to have been made. 



The admirable manner in which the three large volumes have 

 been prepared, within ten years of the appointment of the present 

 Director, reflects the highest credit upon Mr. Worthen and his 

 assistants, and the authors' work may fairly rank side by side with 

 that of the veteran State Geologist of New York, Mr. James Hall, 

 whose greater labours have extended over a far longer period. 



The State of Illinois embraces a geographical area of about 55,400 

 square miles, and is bounded on the north by the State of Wisconsin, 

 on the east by Lake Michigan, the State of Indiana, and the Wabash 

 Eiver, on the south by the Ohio, and on the west by the Mississippi. 

 For nearly three-fourths of its entire circumference it is bounded by 

 navigable waters, which afford facilities for the cheap transport of its 

 products equalled by few of the neighbouring States and surpassed 

 by none. 



The slope of the watershed is mostly to the south-west, and 

 nearly all the principal streams after a general course in that direc- 

 tion empty into the Mississippi. The face of the country is generally 

 level or gently rolling, but along its northern limits, between Free- 

 port and Galena, the elevations known as the "Mounds" attain a 

 height of 1150 feet above the sea. These mound-like elevations are 

 very common on the prairies in some portion of the State, and 

 appear to be " outliers" resulting from the denudation of the original 

 surface. In the southern portion of the State, on the contrary, a 

 mountain ridge exists, which has resulted from the dislocation and 

 upheaval of the strata from beneath forming an axis of elevation. 



Of the different Geological systems, only the following are re- 

 presented in Illinois, namely, Quaternary, Tertiary, Carboniferous, 

 Devonian, and Silurian ; thus the Cretaceous, Jurassic, and Triassic 

 formations have here no representatives, and it is not improbable 

 that the entire area was above the ocean level during these epochs. 



A section is devoted to Surface-Geology, including the Drift, Loess, 

 and Alluvial Deposits, followed by a chapter on Tertiary Deposits 

 and Coal-measures, from which it appears that the Coal-bearing 

 strata cover more than two-thirds of the surface of the State. The 

 lower coal-seams are restricted to the central and southern portions of 

 the State, while the upper seams extend to the northern confines, and 

 are the ones mainly to be relied on in that part for the supply of coal. 



This distribution of Coal-measures appears to have resulted from 

 a gradual subsidence of the entire surface of the Illinois Coal-field 

 during the Coal-era, from which cause each succeeding division of 

 the Coal-measures was extended in a northerly direction, so as to 

 cover a larger area in that direction than the one which had pre- 

 ceded it. Prof. Lesquereux devotes a chapter to the consideration 

 of the formation of Coal and its distribution in all the various Coal- 

 fields of Illinois. By the same author is an admirable chapter on 

 the Origin and Formation of Prairies, the chief facts of which were 

 published by him in 1857. 



Prof. Lesquereux has further enhanced the value of these volumes 



