114 PROCEEDINGS OF WASHINGTON MEETING. 



portion of the Coal Measures, where they occur near the surface and where they 

 have heen principally and most extensively operated up to the present time. They 

 seem here, however, to be mure irregular in character and distribution than in the 

 interior, so far as one can judge from the limited developments which have been 

 made in the deep-seated coals of the interim- region. 



Among the most noticeable features of the stratigraphy of these Coal Measures 

 is the variability of details. The strata are characteristically non-persistent, as 

 regards thickness as well as material. Beds of coal thin out and disappear; beds 

 of shale pass into sandstone or grade into limestone, as the case may be ; limestone 

 beds fluctuate greatly in thickness, or may be present or absent in not widely sep- 

 arated localities. These conditions are particularly prevalent over the marginal 

 area, among what has been considered the lower Coal Measure rocks. Swallow,* 

 Norwood,! and Broadhead J all refer to such variations of sections, and they are 

 encountered in mining operations, often to an embarrassing extent. Of most con- 

 spicuously irregular distribution are the sandstones of the marginal area. These 

 sandstones may lie divided into two classes: First, there are the regularly inter- 

 stratified beds, ranging from two to ten or more feet in thickness, which, though 

 less persistent than the other beds, can yet be recognized clearly as interstratified 

 members over considerable areas. Second, there are the great massive deposits of 

 sandstone, sometimes exposed to a thickness of 50 or 60 feet without displaying 

 any bedding planes. These may be connected with the thinner interstratified 

 beds, but where they attain their characteristic development they cannot be 

 classed as interstratified beds of the Coal Measures, but apparently are deposits 

 tilling channels which were eroded in the Coal Measure strata presumably during 

 the Coal Measure period.;; 



The fauna of the Coal Measure rocks indicates the previous existence over the 

 marginal area, in what have been termed the lower Coal Measures rocks, of brackish 

 and shallow waters, while in the interior, among the rocks designated upper Coal 

 Measures, marine forms are more abundant. There is nothing at all pronounced 

 in the fauna which would call for great priority of deposition of the rocks of the 

 marginal area over those of the interior. 



l &* 



The Conditions of Deposition. 



From a consideration of the facts and conclusions presented in the preceding 

 pages, it appears that the following conditions must be satisfied by any interpreta- 

 tion of the process of deposition which may be offered : 



1. That the marginal conditions were generally those of brackish water and 

 favorable for the formation of the coal beds. 



2. That marine and deep-water conditions were more frequent over the central 

 area, permitting the deposition of thick beds of limestone. 



* Report Mo. Geol. Sur., 1855, p. 87. 



f Report Mo. Geol. Sur., 1873-74, pp. 200-215. 



1 Report Mo. Geol. Sur., 1872, part ii, p. 166, and elsewhere. 



gThese channel deposits are. in places, a mile or more wide and apparently 200 or more feet 

 thick; they limit sharply the coal beds and the other regularly deposited strata. Their distribu- 

 tion is being carefully studied by the state geological survey, and they promise to prove a most 

 valuable and interesting subject of study. Their exact age is not at present determined, and it is 

 possible that they may ultimately be assigned to tie- Permian or even to a later period < m the 

 other hand, if they ••an he traced beyond the limits of the Coal Measiires.it i> probable that at 

 [east a part of the sandstone which has been classed as the Ferruginous sandstone of pre-Coal 

 Measure age really belongs to this formation. 



