CORRELATION OF SECTIONS 187 



This leaves the Stniwn, Milsap, and perliaps a })art of tlie HiJiul 

 division etiuivalent to the Des Moines and Arkansan series. The great 

 tliickness of these Texas divisions seems to give support to this inter[)re- 

 tation. The revision of Drake's correlation^' of the Indian Territory 

 Coal Measures with tliose of Kansas corroborates this opinion. 



CORRELATION OF WESTERN INTERIOR AND ALLEGHANY SECTION 



Without more exact stratigraphic knowledge than at i)resent exists 

 regarding the vast intervening area, all attempts at correlation of the 

 sections of the two great fields can only be considered as merely broad 

 approximations ; yet the comparison is not without interest and value. 

 The ])rincipal data has recently been gathered from the plant re- 

 mains. 



If we follow the suggestions of David White, the Des Moines series in 

 western Missouri is to be regarded as representing the Alleghany series 

 of Pennsylvania."!" In the consideration of the fossil plants of the 

 McAlester coal field by the same author ;j; three widely separated horizons 

 are represented. The middle one, the McAlester coal, is regarded as be- 

 longing to the level of the lower part of the Missourian series, and as 

 probably older than the Pittsburg coal, the base of the Monongahela 

 series of Pennsylvania. The lower horizon, the Grady coal, is placed in 

 the Alleghany series, and " probably in the lower half." 



According to the evidence now presented both the McAlester and 

 Grady coals are in the Des Moines series. 



Unconformity at Base of Coal Measures 

 extent and nature 



West of the Mississippi river the unconformity at the base of the Coal 

 Measures is known to extend in a north-and-south direction a distance 

 of more than 500 miles. This is the distance from about the north 

 boundary of Arkansas to the southern limit of Minnesota. 



From the Mississippi river the rocks have a general dip to the west- 

 ward. Over a considerable belt of country west of the great river the 

 juncture of the Coal Measures with the underl3^ing formations is visible. 

 The width of this belt is from 100 to 200 miles. How much farther west- 

 ward it extends is not known, since the horizon soon is covered too deeply 

 by the overlying strata. 



* Proc. Am. Philos. Soc, vol. xxxvi, 1897, p. 388. 



fU. S. Geol. Survey, monog. xxxvii, 1899. 



X U. S. Geol. Survey, Nineteentli Ann. Rept., pt. iii, 1900, p. 457. 



