REPRESENTATION OF GEOLOGICAL FORMATIONS 695 



other is formed by the basal sandstones of the Coal-measures. 

 Farther southward, in Arkansas, the difference in the strati- 

 graphic horizons of the two is upwards of 20,000 feet. Yet, 

 because of peculiarities of position, the existence of an interven- 

 ing unconformity plane, and the nearly same level above the 

 sea of neighboring outcrops, of the two horizons on the Missis- 

 sippi river, Worthen 1 and others were led to erroneously ascribe 

 to the Kaskaskia (Lower Carboniferous) an extensive flora of 

 the Coal-measures. 



A different example is that of the Carboniferous of Arkansas. 

 There is the enormous thickness of 26,000 feet of sediments. 

 Sedimentation has been uninterrupted throughout the entire 

 sequence. In the last formed terrane of the Lower Carbonif- 

 erous, there begins an alternation of sandstones and shales, 

 with some coal seams, continuing to the top of the section. 

 About 24,000 feet of this section may be regarded as a litho- 

 logical individual quite "uniformly varied in character." Data 

 obtained farther north in Missouri show that 23,000 feet of 

 this enormous section are unrepresented. The Arkansas sec- 

 tion belongs to at least three great terranes, each having a tax- 

 onomic rank of series. Measured in feet, the median one alone 

 is five times as great as all the rest of the Carboniferous repre- 

 sented in the Continental Interior. The conditions presented 

 are represented below. Viewed from Arkansas alone the lines 

 separating the distinct 

 geological formations 

 might forever remain 

 unnoticed in the great 

 lithologic individual. 

 It is only by a com- 

 parison with sections in other localities that the terranal 

 divisional lines may be properly drawn. 



There seems to be only one answer to the question: "What 

 should a geological map represent?" That is Mr. Cross' obser- 

 vation that " it should represent as much of the geologic develop- 

 ment of the earth recorded in the area covered as is practicable." 



1 Illinois Geol. Surv., Vol. I, p. 79, 1866. 



<&'/, 



Fig. 2. — Carboniferous Sedimentation in Missis- 

 sippi Valley. 



