284 C. R. KKYES CARB0XIFP:R0US strata of central IOWA. 



the great thickness of sandstone was probably almost entirely removed in 

 places. 



While no other arenaceous bed, within the limits of the area under con- 

 sideration, is so extensive as the " Redrock " sandstone, there are numerous 

 sandy strata of considerable importance ; yet they contain a large per- 

 centage of clayey material and are more or less distinctly laminated, thus 

 partaking more of the character of sandy shales. 



Passing westward, the sandrocks appear to gradually acquire more and 

 more clay, becoming first shaly sandstone beds, and ultimately homogeneous 

 clay-shales with no grit whatever. Another noticeable feature in this gradual 

 transition from a compact massive sandstone to a clayey shale is that the 

 bed becomes thinner as the proportion of clay increases, so that, when the 

 stratum has changed its facies completely, the argillaceous layer is perhaps 

 only one-third or one-fourth of the maximum thickness. 



Calcareous Beds. — The limestones of the Coal Measures play an unim- 

 portant part in the lithological features of the region ; they consist merely 

 of a few thin bands in the upper portion of the general section, i. e., above 

 the Lower Coal Measures, as commonly designated in this part of the state. 

 Though seldom exceeding ten or twelve inches in thickness, these calcareous 

 bauds are the most persistent and easily recognizable, over wide areas, of 

 any of the horizons in central Iowa. They are fragmentary or nodular, 

 very impure from a large admixture of clayey material, and more or less 

 highly fossiliferous. 



Coal. — From an economic standpoint, the coal of the region forms by far 

 the most important deposit. The seams vary from a few inches to seven or 

 even eight feet in thickness ; the average of the veins at present worked be- 

 ing between four and five feet. These are disposed, not in two or three con- 

 tinuous layers over the eutire area, but in numerous lenticular masses from 

 a few hundred yards to several miles in diameter. A single horizon may 

 thus contain several of these lens-shaped beds of greater or less extent. 

 Along the line of the general section the coal-bearing horizons have been 

 found to number more than a score ; and the extension of the investigations 

 beyond the limits of the particular area here considered has very greatly 

 increased this figure. Recognizing this fact, the aggregate amount of coal 

 is far in excess of what has been supposed hitherto. The peculiarities of 

 its disposition and the consequent popular misunderstanding concerning the 

 actual extent and distribution of the coal beds has led to a large but useless 

 expenditure of capital. This phase of the question will receive further ex- 

 pansion in another place. 



