PENNSYLVANIAN STRATIGRAPHY OF TEXAS 39 



south of the sand to the border of the Llano Mountains southwest 

 of Brady. In the northern Pennsylvanian area its thickness is 

 about 150 to 200 feet, in the southern, 100 to 125 feet. The forma- 

 tion has not been subdivided into named members, but three of 

 the Hmestone beds which have been separately mapped for long 

 distances have been differentiated. The Breckenridge limestone, 

 3 to 5 feet thick, occurs at the top, the Black Ranch limestone, 

 about the same in thickness, is found 20 to 40 feet below the top, 

 and the Ivan limestone, 50 to 80 feet below the top. The two lower 

 beds have not definitely been identified in the Colorado Valley, 

 though they are possibly equivalent to Drake's "Lower Chaffin" 

 and "Speck Mountain" limestones. 



The Harpers ville formation is named from a small town 10 

 miles south of Breckenridge in Stephens County. It includes the 

 strata from the top of the Breckenridge limestone to the top of the 

 Saddle Creek limestone of Drake, and is chiefly characterized by 

 its content of one of the important workable coals in the north 

 Texas Pennsylvanian, "Coal No. 6" of Texas reports. The shales 

 which are the chief constituent rock are carbonaceous and ferru- 

 ginous, containing siderite and limonite concretions, and locally 

 abundant, well-preserved fossils. The only important limestone in 

 the south is the Saddle Creek member, which consists of two or 

 three beds i to 3 feet in thickness separated by shale; but in the 

 north a second limestone, named the Belknap, 30 to 50 feet below 

 the top of the formation, is traceable for a long distance; and in 

 the vicinity of Crystal Falls, Stephens County, a third bed, the 

 Crystal Falls limestone, occurs 60 to 80 feet lower. The total 

 thickness of the Harpersville formation is 200 to 275 feet. The 

 fauna is a fairly large one, but it is very unlike that of the lower 

 portion of the Cisco described in the Graham. The species most 

 common there are here missing, and new species, as Enteletes 

 hemiplicata, Chonetes granulifer meekianus, Derbya cymbula, and 

 other forms, appear. Also species which are present in the lower 

 rocks as a rather unimportant faunal element are here very abun- 

 dant. The fauna of the Harpersville is indicative of a late portion 

 of the Pennsylvanian. It is approximately equivalent to the 

 Douglas or Shawnee formations of the Kansas section. 



