460 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Ouachita region seems to involve the fewest inconsistencies. This conclusion is tentative 

 and unsatisfactory but seems to accord best with the evidence known to me. Future discoveries 

 may prove it wrong." 



The Pennsylvanian shales, limestones, and sandstones of Oklahoma have been 

 studied by Gould and his associates/'^ who have classified them as follows: 



The rocks in the area under discussion consist of a thickness of from 10,000 to 12,000 feet 

 of sediments of Pennsylvanian age extending from the Mississippian to the Permian. In gen- 

 eral these rocks consist of alternating layers of shale and sandstone. The shales greatly predomi- 

 nate, making up approximately three-fourths or possibly nine-tenths of the entire thickness 

 of the strata. In the northern part of the State there is a number of ledges of limestone, but 

 even in this region shales constitute more than three-fotirths of the strata, the remainder being 

 about equally divided between sandstone and limestone. Farther north, in east-central Kansas, 

 the limestone ledges are not only more numerous (no fewer than twenty distinct ledges having 

 been recognized) but they are thicker, while sandstones make up a relatively small per cent of the 

 entire series. 



In Kansas it has been noted that on passing from the center of the State to the Oklahoma 

 line the various limestone ledges frequently thin out and some of them disappear before the line 

 is reached. Others pass for several miles into Oklahoma before disappearing, but, except iu 

 eastern Kay County, comparatively few limestones reach the Arkansas River. Of the half 

 dozen or more which persist south of that stream only one ledge, so far as known, exceeds 10 

 feet in thickness. 



It has also been found that as the limestones thin out to the south and finally disappear, 

 sandstones often come in, usually either just above or just below the limestone. In other cases 

 the ledges become more arenaceous to the south, until finally the ledge which was a limestone 

 has become a sandstone. It frequently happens, also, that additional sandstone ledges come 

 in, first as mere arenaceous bands in the shales, then as thin lenses which thicken to the south 

 and finally become ledges of hard sandstone 20 to 50 feet thick, which resist erosion and give 

 rise to pronounced escarpments. In a number of instances, however, the thinning out of the 

 limestone ledges is to the north. This fact is particularly well exemplified in the case of several 

 limestone ledges in the region about Bartlesville, and between that city and Arkansas River 

 above Tulsa. Here at least two heavy ledges of limestone disappear to the north. Many of the 

 ledges, both limestone and sandstone, when traced for any considerable distance are found to be 

 local lenses. The general rule throughout the area is, however, that limestones "finger out" 

 to the south and sandstones to the north, while the intervening shales coalesce. 



Another factor which complicates the difficulty of the situation is the gradual thickening 

 of the entire series to the south, particularly near their base. For instance, the basal group, 

 to be hereinafter described as the Muskogee group, is about 450 feet thick at the Kansas- 

 Oklahoma fine, but in the region north of the Choctaw fault, in the eastern part of the State, the 

 same group has an approximate thickness of 9,000 feet. Part of the apparent thickening in 

 this instance is probably caused by unconformity by overlap on the Mississippian, but much of it 

 is doubtless caused by normal thickening of strata southward. The thickening of other groups 

 from north to south, while not so conspicuous as in the case mentioned, is nevertheless often 

 obvious. 



From what has preceded, it will be evident that the problem of the proper division and 

 correlation of the various beds is, in the absence of paleontological data, at best difficult. In 

 rare cases only may any single ledge of either limestone or sandstone be traced from Kansas 

 line southward for any considerable distance. As has been stated, many of the sandstones 

 and a few of the limestones are lenticular and have but a comparatively limited areal extent. 

 These conditions have already given rise to considerable confusion in folio and economic map- 

 ping, and it is feared that, as this work progresses, the complications may be augmented. It 

 is with the hope of lessening the possibility of confusion in the application of stratigraphic terms 

 and the needless multiplication of names in the future that this article has been prepared. 



