428 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Section of Marble Falls limestone along nortJi hank of Colorado River from dam, to point above 



bridge at Marble Falls. 



Smith wick shale. Feet. 



Thin-bedded black limestone 20 



Fine-grained dark-gray limestone 15 



Thin-bedded and massive black limestone 30 



Brownish-gray sub vitreous to crystalline limestone 30 



Thin-bedded dark-gray or black fossiliferous limestone with Productus and crinoids 4 



Fine-grained crystalline gray limestone 4 



Fine-grained gray limestone with molluscan fossils 20 



Dense black or dark-gray limestone 2 



Light and dark gray mottled limestone with chert nodules 5 



Mottled gray limestones with crinoid stems 26 inches long 5 



Limestone conglomerate 1 



Gray limestone without chert 2 



Gray cherty limestone 8 



Black limestone with cherty layers 8 



Black, evenly bedded limestone with cherty layers 9 



Massive gray crystalline limestone 8 



Gray cherty limestone 25 



Gray crystalline lim.estone 6 



Fault. 



Irregular-bedded gray limestone 17 



Black shaly limestone with chert layers and lenses 22 



Massive gray limestone; crinoid stems 60 



Fault. 



Dark cherty limestone 7 



Dove-colored limestone with black bands and lenses 40 



Conglomerate 20 



Ellenburger limestone (Cambrian and Ordovician). 



868 



The Smithwick shale consists of a soft, very dark to nearly black carbonaceous shale irt 

 which are included a number of sandstone lentils. Because of its soft nature the formation 

 was not observed in such attitude as would permit its measurement. Moreover, its top is 

 overlapped by Cretaceous sediments. Probably the beds exposed in Burnet County do not 

 exceed 400 feet in thickness. The basal boundary of the formation is always sharp, the change 

 to limestone occurring very abruptly. The formation represents the youngest Paleozoic sedi- 

 mentation in the region and the erosion surface, represented by its beveled beds, marlcs th& 

 great unconformity between Paleozoic and Cretaceous sedimentation. The formation is 

 confined to the southeast portion of the Burnet quadrangle and to a small area in Riley Moun- 

 tain, in the Llano quadrangle. 



I 14. NORTHERN TEXAS. 



Northern Texas is geologically almost completely separated from the areas to- 

 the north by the uplift of the Arbuckle Mountains. There is a possible connection 

 around the west end of this uplift by "Red Beds" strata as yet unclassified. The 

 stratigraphy of the Pennsylvanian of northern Texas was summed up ,by Taff ^^ 

 as follows : 



The north Texas coal field extends from the south side of the Colorado River valley between 

 Lampasas and Concho counties northward to Red River in Montague County. It is nearly 

 250 miles in length, with an average width of about 45 miles, and has, therefore, an approxi- 

 mate area of 15,000 square miles. * * * 



The north Texas coal field is limited on the east by the Cretaceous rocks, which rest uncon- 

 formably upon Coal Measures. At the south end of the field the Coal Measures lie unconform- 

 ably upon the Mississippian (Lower Carboniferous) . and are in part concealed by Cretaceous 

 strata. Upon the west and northwest sides the Coal Measures are bounded and succeeded. 

 stratigraphically by the Permian. * * * 



