IOWA ACADEMY OF SCIENCES. 2T 



extent of at least twenty miles; and probably stretches out much farther. At Red- 

 rock Cliff the stone is massive for the most part, but rather soft and thin-bedded 

 above. At this place it is a very fine grained and horaopeneous sand rock, some 

 portions even affording excellent material foi grindstones. But southeastward, and 

 at Elk Bluff, two miles below, the sandstone passes into a fine-grained, ferrugin- 

 eous conglomerate. The dip is everywhere to the south and west; and at a short 

 distance above the quarry, a short distance above the village, the mclination is very 

 considerable. A mile beyond, the sandstone has disappeared completely and the 

 section shows only shales and clays. The space between the latter exposure and 

 the last known outcrop of the sandstone is perhaps half a mile, the interval being 

 hidden by quaternary deposits down to the water level. The abrupt change in the 

 lithological characters of the rocks in so short a distance has been mentioned by 

 Owen and by Worthen; but the true explanation is entirely different from the 

 suppositions of those writers. 



Recent observations have cleared up many of the hitherto doubtful points con- 

 cerning the geological history of the Redrock sandstone. It is not the basal 

 member of the coal measures, as was regarded by Worthen; nor is it a shore 

 extension of the Kaskaskia limestone; neither is its geographic extent as limited as 

 has been supposed. Twenty miles to the southeast of Redrock a sandstone of 

 great thickness, having identical lithologic characters and with a similar strati- 

 graphical position is believed to be its extension southward. And it may also 

 rise a few feet above low water in the northwestern corner of Marion county. The 

 most interesting consideration in regard to this Redrock sandstone is the fact of 

 its considerable elevation above the surface of the sea and its subjection to 

 subaerial erosive agencies for a long period of time before submergence again took 

 place. During that interval the great thickness of sandstone was probably almost 

 entirely removed in places. 



GEOLOGICAL STRUCTURE AND RELATIONS OF THE COAL-BEARING 

 STRATA OF CENTRAL IOWA. 



BY CHARLES R. KEYES. 



The exposed stratified rocks of central Iowa are made up chiefly of Lower Coal 

 Measure clays, shales and sandstones. In the southeastern portion of the area 

 the upper member (for Iowa) of the Sub-Carboniferous— the St. Louis limestone- 

 is exposed along the Des Moines river. To the westward the so-called Middle 

 Coal Measures and the Upper Coal Measures are represented. Hitherto it has 

 been supposed that the three recognized divisions of the upper Carboniferous 

 rocks in the State have each a maximum thickness of about two hundred feet. 

 Lately, however, the Upper Coal Measures alone have been discovered to have at 



♦ Published iu full in the Bulletin of the Geological Society of Americii. Vol. II. pp. 

 277-292. pis. ix, x. (1891.) 



