PENNSYLVANIAN. 429 



The Coal Measures of north Texas were separated by ]\Ir. Cummins ° into five divisions 

 or groups, as follows, in descending order: Albany, Cisco, Canyon, Strawn, MiUsap. 



Millsap division. — This group of strata has not been recognized south of northern Erath 

 County, though, according to the structure of the rocks, it should occur in the basin of Leon 

 River in Comanche County. According to the Texas Survey it thins southward and does 

 not occur in the Colorado River valley. North of Trinity River valley in Wise County it is 

 concealed by the overlap of Cretaceous rocks. The top of the formation includes coal bed 

 No. 1, whose outcrop is located upon the map and which is mined at Thurber, Gordon, Rock 

 Creek, and Bridgeport. The strata of this group occur at the surface between the coal outcrop 

 referred to above and the Cretaceous border. The base is not exposed, and much of the section, 

 which has a thickness of about 1,000 feet, is concealed. The complete section was obtained 

 by diamond drill, penetrating the strata at Thurber. 



It is composed of blue and black shale, interstratified with occasional limestone and sand- 

 stone strata. 



The deep wells at Gordon and Thurber produce gas and salt, but no coals were encountered 

 below coal No. 1. 



Strawn division. — This is composed of nineteen separate members of sandstone and shale, 

 making an aggregate thickness of 3,700 feet in the southern part of the field, where the mem- 

 bers of the group rest unconformably upon the massive shale and limestone of the Mississippian, 

 and in many cases are unconformable with regard to each other. 



In the northern part of the field the aggregate thickness decreases to 950 feet. Coal is 

 not known to occur in this division. 



Canyon division. — This group is composed chiefly of limestone and shale, with local beds 

 of sandstone and conglomerate. The thicker beds of limestone occur near the base and in 

 the upper part of the series. The Hmestones are generally thick or massive and hard, making 

 escarpments or outcropping in prominent ledges. In the northern part of the field the group 

 contaiijs twelve separate members of limestone and shale and one conglomerate, making an 

 aggregate in all of 800 feet of strata. Unlike the Strawn, this division increases in thickness 

 northward. In the northern part of the field it contains 930 feet of strata. No coal is reported 

 to occur in these rocks. 



Cisco division. — There are nineteen different members of this group, each of which is found 

 to be variable in lithologic nature. Some of the members contain limestone, shale, coal, and 

 sandstone, and several contain conglomerate, while a few are composed almost entirely of 

 limestone and shale. Throughout the section there are not fewer than twenty separate beds 

 of limestone, which are usually thin. The shale as a whole is much thicker than all the other 

 strata together, and the whole group aggregates about 800 feet from end to end of the field. 



Several of the members which contain conglomerate rest upon eroded surfaces of lower 

 beds and in some instances have been found to overlap and displace beds of coal. The hetero- 

 geneous nature of this group of strata indicates many repeated changes in sedimentation. 

 Several thin beds of coal have been recognized near the top and in the lower middle part of 

 the group, besides two beds of workable thickness. These workable beds occur 200 and 300 

 feet above the base of the division. 



Albany division. — The class of sediments making this group is generally a repetition of 

 that found in the Canyon division. They are thick beds of limestone generally separated by 

 shaly members. 



According to Mr. Cummins's report the Albany division thins northward from 1,180 feet 

 at the south end of the field to zero on the Brazos River. 



It contains two shaly members which bear thin coal strata, none of wliich, however, have 

 been found to exceed a few inches in thickness. 



This group is succeeded by the Permian, which overlaps upon the Cisco division between 

 Brazos and Red rivers. 



"Cummins, W. F., Report on the geology of northwestern Texas: Second Ann. Kept. Texas Geol. Survey, 1890. 



