INTERGRADATIOX OF (^LAYS AND PAXDS. 283 



place laterally in the same horizon, or vertically from one layer to another. 

 By a constant increase of carbonaceous matter the dark-colored shales 

 become highly bituminous and then coaly. 



The economic aspects of the Carboniferous clays and shales in the area 

 under consideration need not be dwelt upon here. They are of such great 

 practical importance that they form the theme of a special investigation, 

 the results of which will soon be announced. 



Arenaceous Deposits. — Although a large amount of sandy material is 

 present in the Coal Measures of the region under consideration, it is usually 

 mixed with clay to such an extent as actually to form sandy shales. In 

 some cases, however, the sand constitutes a sandrock which is sufficiently 

 compact to afford material for ordinary rough masonry. The hard 

 portions of the sandstones are for the most part very limited, being only 

 two or three feet in thickness ; or in the form of large spherical concretions 

 in a softer matrix. These sometimes attain a diametric measurement of 

 five or six feet. Within the limits of the area in question there is, however, 

 one notable exception to the general character of the arenaceous deposits, 

 i. e., the " Redrock " sandstone. This is an enormous consolidated sand bed 

 having a geographic extent of more than twenty miles in its longest direc- 

 tion and at least six or seven miles in width. It rises in high mural escarp- 

 ments along the Des Moines river, chiefly in Marion county. For the most 

 part it is a rather compact, massive, homogeneous sandrock, though in places 

 it passes into a fine-grained ferruginous conglomerate. Occasionally large 

 spherical concretions are met with. In the upper part it becomes thinly 

 bedded, with a considerable amount of clay intermingled. The base is rich 

 in plant remains : lepidodendrids, sigillarids, calamites and ferns of many 

 species. The upper surface has been subjected to sub-aerial erosive agencies, 

 as has been fully shown in another place."*" This denudation took place 

 during the Lower Coal Measure period, since an extended coal deposit, with 

 its associated shales, is found laid down in the old gorges of the sandstone. 



In regard to this remarkable sandrock formation, the following conclu- 

 sions have been reached, as fully stated elsewhere : f 



(1.) It is not the basal member of the Coal Measures, as it was considered 

 by Worthen. 



(2.) It is not the shore extension of the Kaskaskia limestone, as was at 

 one time supposed. 



(3.) Its geographic extent is not so limited as it has been regarded. 



(4.) The most interesting consideration is the fact of its elevation above 

 the surface of the sea and its subjection to atmospheric influences for a long 

 period of time before submergence again took place. During that interval 



* Am. Journ. Sci, 3d series, vol. XLI, 1891 (iu press). 

 i Loc cit. 



