422 Beviews — Foster's Mississippi Valley. 



Turning now to the geological structure of the valley, the basin 

 consists for the most part of nearly horizontal and but little meta- 

 morphosed strata, such as limestones, sandstones, shales, and detrital 

 materials bounded by granitic or highly metamorphosed rocks, which 

 have at many points been invaded by products of a purely volcanic 

 origin. 



It is chiefly in this latter series that the precious as well as the 

 baser metals occur, either segregated as veins or as beds or layers. 

 Not that they are entirely confined to this series, for the stratified 

 rocks, as the limestones and shales, contain also the useful ores, such 

 as iron, lead, and zinc, as well as immense deposits of fossil fuel 

 extending over vast areas. 



The author gives (at p. 246) a tabular view of the principal Fossili- 

 ferous strata of the Mississippi Valley and Pacific slope, in which 

 local terms are given for the sub-divisions of the groups. Thus, for 

 example, in the Carboniferous series we have in descending order — 

 1. Coal-measures ; 2. Chester Limestone ; 3. St. Louis group ; 4. Keo- 

 kuk group ; 5. Burlington Limestone ; 6. Kinderhook group ; which 

 vary from 800 to 1400 feet in thickness exclusive of the Coal-measures, 



The Burlington Limestone is celebrated, more especially in Illinois, 

 for the beauty and profusion of its Crinoids. Mr. Worthen (Geolo- 

 gist to the State of Illinois) writes, " No spot has yet been discovered 

 on the surface of the earth, of the same extent, where those beautiful 

 ' lily-stars' flourished in such numbers, as along the northern shores 

 of the Sub-Carboniferous ocean during the deposit of this limestone. 

 More than 300 species have been already described from this region." 



The vast area in the great valley over which the Coal-measures 

 are distributed is divided into several distinct fields, intersected by 

 navigable waters, by which their products are conveyed to the great 

 emporiums of industry. 



The Alleghany Coal-field ranges through six different States, and 

 has an estimated area of 60,000 square miles. The shales, lime- 

 stones, etc., have a thickness of 2,500 to 3,000 feet. The workable 

 coal-seams at Pittsburgh have an aggregate thickness of 25^ feet, 

 and in Southern Ohio of 22^ feet. 



The Illinois Coal-field is, in area, about equal to the Alleghany 

 Coal-field. The Coal-measures in this field have a thickness of 800 feet, 

 and the workable coal in Southern Illinois is 19 feet in thickness. 



The Missouri Coal-field has the largest area of any field in the 

 world, being not less than 100,000 square miles in extent. In 

 Kansas the series is two thousand feet in thickness, and contains 

 from 12 to 15 feet of workable coal. 



The Michigan Coal-field has an area of 5,000 square miles, and a 

 thickness of 100 feet only. 



The Texas Coal-field has only been very imperfectly explored, and 

 neither its extent or thickness are correctly known. 



The coals derived from these various fields, and even from 

 different seams, are far from uniform in character. The most 

 valuable coals perhaps, thus far developed, are those of Northern 

 Ohio and North-western Pennsylvania derived from the lowest seam 



