LiTHOLOGICAL FEATURES OF THE StKATA. 



General Characters. — In lithological characters the Coal Measures of cen- 

 tral Iowa contrast sharply with the other Paleozoic formations of the state. 

 Not less striking is the relative thinness, as a rule, of the individual layers, 

 or beds, which follow and replace one another, upwards and laterally, in 

 rapid succession. Often within a vertical distance of a few inches or a few 

 feet, layers of sand, clay or shale are succeeded by different strata ; or else 

 are changed both in color and chemical composition. Of the three general 

 types of rocks recognized, the argillaceous are the most prominent and most 

 widely distributed ; arenaceous deposits are developed only in much less 

 volume ; while the calcareous rocks are exceedingly unimportant and are 

 restricted to a few thin bands, seldom more than eight or ten inches in maxi- 

 mum thickness. 



Argillaceous Materials. — The clay-shales make up by far the greater por- 

 tion of the Lower Coal Measures in Iowa. On exposure to atmospheric 

 agencies they quickly disintegrate into soft clays and are easily carried away 

 by running Avater. For the most part they are ashen, drab, or black in 

 color, though red, yellowy buff and blue shades are of not uncommon occur- 

 rence. In some localities the variegated shales — blue, drab, red, yellow and 

 ashen indiscriminately mingled — predominate. It is in the latter shales that 

 crystallized gypsum frequently occurs abundantly. At Des Moines and else- 

 where, diamond-shaped crystals of selenite are the more plentiful, though not 

 infrequently some of them are greatly elongated in the direction of the ver- 

 tical axis, sometimes attaining a length of eight or ten inches. In the latter 

 habit, twinning is quite common. Often the crystals are acicular and, radi- 

 ating from a center, form little rosettes, which lie in great numbers on the 

 exposed surfaces of clays. 



The light-colored shales occasionally afford impressions of ferns and lepi- 

 dodendron roots, but for the most part they are unfossiliferous. The dark- 

 colored, bituminous varieties, on the other hand, are often highly charged 

 with organic remains. From a single locality (Des Moines) nearly one 

 hundred species of invertebrates have been recognized, besides a number of 

 fossil fishes and plant remains. A partial list of these organisms, with full 

 notes, has been given in another place,* and considerable additional infor- 

 mation of the same sort will soon appear in a form for reference. 



The light-colored shales, by the gradual addition of fine, sandy material, 

 pass imperceptibly into sandy shales, and these again into shaly sandstones 

 and finally into hard, compact sandrock. This gradual transition may take 



*Proc. Acad. Nat, Sci., Phila., 1888, pp. 222-246. 



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