CENTURY OF IOWA GEOLOGY 427 



SCHEMATIC STANDARD FOR AMERICAN CARBONIC ROCKS 



As the Iowa Carbonic section began to expand it became mani- 

 fest that there were considerable portions of it which were to be 

 found in better representation elsewhere. When, finally, certain 

 guide-horizons were traced from outcrop to outcrop southward 

 across Missouri into Arkansas and southwestward into Kansas the 

 section gradually assumed extraordinary completeness. It resolved 

 itself into what was the most perfect section of the entire country, 

 perhaps of the world. This fact at once pointed out its possible 

 availability as a standard scheme for the American continent.-' 



Nine-tenths of this great section are represented in our state only 

 by an unconformity plane ; yet taking into account its southern ex- 

 tension its chief merit is its exceptional completeness. No other 

 section of the Carbonic rocks on the continent possesses such an 

 enormous thickness. The several series of the general succession 

 appear to be more sharply defined than anywhere else. Few col- 

 umns have the base so abruptly cut off from the Devonic terranes 

 below. At the top Cretacic strata often rest upon it in marked un- 

 conformity. Instead of attempting to fit the Mississippi valley sec- 

 tion into that of Pennsylvania, as is usually done, effort is far more 

 fruitful of satisfactory results by bringing the rock succession of the 

 latter into accord with the former. Also, in place of trying to ex- 

 tend the Mississippi basin classification to the Rocky Mountain 

 region the formations of the latter are best apposed to those of 

 the first mentioned. 



Until recently, more or less difficulty in securing results that 

 are even approximately satisfactory have always attended the efforts 

 to parallel the various provincial sections of the Carbonic succession. 

 The sections of the East, of the Interior, and of the West appear at 

 first glance to have no comparable elements. These discrepancies 

 are now all removed. The out-standing features are the complete- 

 ness and great thickness of the Mississippi Valley succession. It is 

 most imposing rock section of which we know. 



Apparently influenced mainly by such consideration as these 

 Professor Chamberlin is inclined to emphasize the time significance 

 of our coal measures depositions and in the general classificatory 

 scheme give the span they occupy an exalted taxonomic rank equiva- 

 lent to Period. There are, however, essentials other than those of 

 thickness. The mere fact that the column of sediments measure 

 20,000 feet does not necessarily remove the terrane from the serial 

 class. 



= 'Iowa Geol. Surv. Vol. II, Coal Deposits, pp. 162-171, 1894. 



