724 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



cealed or inconspicuous because of the similarity of the Eocene and Cretaceous sediments. In 

 northeastern Arkansas the Midway overlaps the Cretaceous, and rests upon a Paleozoic floor. 



Stephenson and Crider, in an unpublished manuscript, state: 



"In Pulaski County, Ark., the strata of the Midway formation, where most completely 

 developed, are separable, according to Harris," in ascending order into (a) white, compact sand 

 (2 feet); (&) ledge of calcareous sandstone (2| feet); (c) the 'Turritella limestone' (8 feet); (d) 

 the 'Ostrea [Gryphsea] limestone' (3 feet); (e) the 'Enclimatoceras limestone' (IJ feet). In 

 general, the 'TurriteUa limestone' in Arkansas consists of layers of hard light-gray to blue 

 limestone interbedded with layers of sand, and this division is the one most often ceen in out- 

 crops. The formation is easily separable both lithologically and paleontologically from the 

 underlying Paleozoic rocks and the overlying younger formations." 



According to Harris and to Stephenson and Crider, the northernmost Midway outcrop is 

 near Newark, in Independence County, Ark., whence it has been traced southward, more or less 

 continuously, along the edge of the Paleozoic rocks to a point near Rockport on Onoshata River, 

 Hot Springs County. North of the locality near Newark it has not been recognized on the sur- 

 face or in well borings. In southern Missouri the group may be represented by the Porters Creek 

 clay. From Olyphant, Jackson County, to Russell, White County, the Midway outcrops in 

 numerous places along the St. Louis, Iron Mountain & Southern Railway and on the terrace-like 

 elevation which extends almost unbroken from Russell to Olyphant. It occurs in Pulaski and 

 Lonoke counties. The best exposures in Arkansas are between Little Rock and Malvern. It 

 outcrops in Little Rock near the point where the Hot Springs wagon road crosses the St. Louis, 

 Iron Mountain & Southern Railway and may be traced almost continuously from this place west- 

 ward to Malvern. Outcrops of the Midway are not known in Arkansas southwest of Hot Springs 

 County. 



^ In Louisiana two outcrops of the Midway are known, one on the road from MarthavUle 

 to Many, at Rocky Spring Church, in Sabine Parish, and the other at King's salt works, in 

 southwestern BienviUe Parish. These outcrops have been brought to the surface by anticlinal 

 folding. The material in both localities is calcareous.*"'^ Harris *"® says regarding a third 

 probable outcrop: 



"Farther to the south in the Sabine uphft territory — for example, along the Texas & 

 Pacific Railway for over a mile in the vicinity of Marthaville — Ostrea thirsse,^ a very char- 

 acteristic fossil of the basal Wilcox formation, occurs in great abundance. As the strata in 

 that vicinity dip to the south, though but shghtly, it is evident that Midway beds must occur 

 at the surface over a considerable area to the north. Their geographic distribution has not 

 yet been worked out." 



In Texas the basal contact of the Midway has been positively identified at only two locali- 

 ties — on Brazos River 1^ miles above the Falls-Milam county line ^"'^ and on Frio River .''^® 

 At the former locality no erosion unconformity can be seen, notwithstanding, the great fauna! 

 change; at the latter locality there is evidence of such an unconformity. Deussen, in an 

 unpubUshed manuscript, says that northeast of Brazos River "at the base of this formation 

 are usually found bluish micaceous clays or clayey sands with occasional light-yellowish fossil- 

 iferous limestone layers of marine origin. These are succeeded by sandy ledges, resting on 

 wliich are generally found black selenitic clays. The outcrop constitutes the Eastern Marginal 

 Prairies and occupies a narrow belt extending approximately north and south in Van Zandt, 

 Kaufman, Henderson, Navarro, FreestonCj Limestone, Falls, Robertson, and Bastrop counties. 

 Much of the exposure is obscured by materials of later age. This formation in northeast Texas 

 averages a^bout 260 feet in thickness. The dip varies between 1° and 5° SE." 



Southwest of Bastrop County the Midway has not yet been definitely traced in Texas, but 

 evidence obtained by Vaughan along Frio River and the Rio Grande indicates that it extends 

 into Mexico. On Frio River,^^^'' 5 mUes due north of the southern boundary of UValde County, 



a Harris, G. G., Ann. Kept. Arkansas Geol. Survey, 1892, vol. 2, pp. 28, 29. 

 b Through a slip of the pen, Ostrea sellxfomis in the original. — T. W. V. 



