UNITY OP INTERIOR COAL-FIELDS 547 



The detailed comparison of the Eastern Interior and Western 

 Interior sections is rendered easy for the reason that it was 

 found that instead of being separated by a wide belt of older 

 rocks the two are actually continuous. This is made evident 

 by the profound Cap-au-Gres displacement which crosses the 

 Mississippi river near the mouth of the Missouri river. This 

 fault having a throw of 1,000 feet drops the coal measures down 

 so that there is now an unbroken belt of them extending from 

 Indiana to Kansas. In this trough also the basal limestones of 

 the Missourian, or Upper Coal, series are brought nearly 100 

 miles closer together than had been previously suspected. 



The problem of comparing the Eastern Interior and Western 

 Interior sections thus consists merely of matching up, after some 

 little special investigation at critical points, the sequence of beds 

 on the two limbs of a broad arch the crest of which is removed 

 through erosion. In the west, in Missouri and Kansas, the de- 

 tails of the rock succession are more clearly discerned on account 

 of the country there being free from glacial debris. The wealth 

 of detail there deciphered is at once the joy of geologists who 

 work in the region and the dispair of outsiders who are un- 

 acquainted with such refinements of stratigraphical conditions 

 It is the most completely differentiated sectioa of the coal meas- 

 ures in the United States. 



On the other hand the coal measures of Illinois are the 

 longest known and least understood stratigTaphically of any 

 coal deposits on the American continent. Notwithstanding the 

 circumstance that one of the very first discoveries of mineral 

 coal in America was made in 1680 near the present city of 

 Peoria, the Illinois section remains today almost as completely 

 undifferentiated according to modern standards as it was three 

 centuries ago. 



With the exception of an early attempt, when the Permian 

 controversy 'was at its height in this country, to show by the 

 contained fossils that certain beds near La Salle were to be com- 

 pared w'ith the Kansas Permian section {i. e., Permo-carbonifer- 

 ous, or ]\Iissourian) all efforts appear to have been towards estab- 

 lishing correlative relationships with eastern sections. This ten- 

 dency seems all the more pronounced since a concerted move-' 

 ment was inaugurated to take the Pennsylvanian section out of 

 the provincial class and make it the standard succession for 

 the entire continent. In so doing important affinities are wholly 



