PROGRESSIVE OVERLAP 597 



Chonopectus sandstone of Burlington. It rests on the eroded lower 

 Magnesian limestone (Ordovicic) and varies in thickness from a few 

 inches to 70 feet. It is not always black, but sometimes greenish or 

 yellowish, and has thin limy members toward the top. It is immediately 

 succeeded by the Saint Joe limestone (Burlington), into which it is 

 often seen to grade. The connection between the two is an intimate one 

 and represents continuous deposition. This fixes the date of the Eureka 

 (Noel) shales of Arkansas as latest Kinderhook and shows an overlap 

 from the base of the Choteau to the base of the Burlington, during the 

 transgression of the Kinderhook sea from southern Missouri to northern 

 Arkansas. Here the basal black shale takes the place of the basal 

 sandstone of the basal Paleozoic sections; but, like that sandstone, this 

 basal shale rises in the scale with the progress of the transgression. It 

 is hardly questionable that the Black shale represents the reworked 

 residual soil of the old land of Ordovicic limestones in the Missouri- 

 Arkansas section. The shale rests unconformably on various members of 

 the Lower Paleozoic limestones, and in each case derives its mineral 

 character from the bed underlying. According to Ulrich, the shale is 

 found resting only on the Key sandstone (Saint Peter), or the mag- 

 nesian limestone of earlier age (Yellville formation). Where the Black 

 shale is not developed, another formation, the Sylamore, often lies be- 

 tween the Saint Joe and the underlying Ordovicics (Polk Bayou lime- 

 stone). The Sylamore formation consists of a shale with a maximum 

 thickness of 15 feet, succeeded by a sandstone of coarse rounded quartz 

 grains containing phosphate nodules. The shale is sometimes black and 

 then resembles the Noel shale, with which it is generally correlated. 

 Ulrich, however, insists on the Devonic age of this rock, on the strength 

 of some fragmentary fish remains (a mandible doubtfully referred to 

 Dynichthys) which indicate that age, and" of some invertebrate fossils 

 "which tend to corroborate this view." The fossils recorded are "a 

 small Lingula that may be the same as L. spatulata of the Genesee shale 

 of New York, and some conodonts." If this view is correct, there is 

 a pronounced hiatus at the top of the sandstone, for the whole Kinder- 

 hook formation is wanting. The phosphatic pebbles of the Sylamore are 

 sparingly represented in the basal portion of the Saint Joe, which is inter- 

 preted by Adams and Ulrich as the result of reworking. Ulrich states 

 that "the Sylamore formation impresses one as the waste of a near-by 

 shore, and thus agrees, not only in its faunal and physical character, but 

 also in its origin, with the Chattanooga formation as developed in 

 middle Tennessee."* The Noel shale, on the other hand, is correlated 



* George Adams and E. O. Ulrich : Professional paper no. 24, U. S. Geological Survey. 



