THE UPPER TRIASSIC OF WESTERN TEXAS. 9 



"Stratigraphy. — The following classification or grouping is not intended as a cor- 

 relation with any other Triassic beds, but only to apply to the Dockum beds over the 

 area examined. The Dockum may be divided into three beds, though some localities 

 show more, that are more or less well marked. * * * These three main beds are as 

 follows: A lower bed of sandy clay, which is from to 150 feet thick; a central bed or 

 beds of sandstone, conglomerate, and some sandy clay, which is from to 235 feet thick; 

 an upper bed of sandy clay and some sandstone, which is from to 300 feet thick. While 

 these groups represent the different geological horizons over most of the Triassic area, 

 there is nevertheless at some places a thinning out of the one, and a thickening of another, 

 which shows that at the same time the conditions of deposition were somewhat different 

 at different localities. The same geological horizon is, therefore, more or less represented 

 in other beds than that which generally represents it. Then, while these beds do not 

 absolutely represent geological horizons, they do so approximately and are so well 

 marked as to be of stratigraphical value." 



On the east side of the Staked Plains the lower beds are red sandy shales which 

 may be traced from the Canadian River southward to where they disappear 

 beneath the Cretaceous and younger deposits south of Big Springs, in Howard 

 County. These beds shade downward indefinitely into the Double Mountain beds 

 of the Permo-Carboniferous. The author has repeatedly crossed the line, in 

 every county of Texas where it lies, from the Canadian River on the north as far 

 south as the Triassic appears, and has been unable to draw any line that can be 

 used to separate the two formations. In passing from the towns Seymour, Vernon, 

 Haskell, Anson, and Abilene westward to the Triassic at the base of the Plains, 

 the same indefinite boundary is crossed. The red beds grow more shaly, and in 

 general there is an increase in the amount of gypsum, which may occur in beds 

 from a few inches to as much as 3 feet thick, or may be in fine seams running in 

 all directions through the red sandy clay. The same condition is found in the 

 breaks of the Canadian River and in the great canyons which penetrate into the 

 Plains in Randall. Armstrong, Swisher, and Briscoe Counties. Similar conditions 

 prevail wherever these two formations come together in New Mexico, Arizona, or 

 Wyoming and the adjacent States. 



In the vicinity of Dickens, in Dickens County, where the transition beds are 

 beautifully exposed in the Croton Breaks, 2 miles directly east of the town, the 

 intermediate series is terminated above by a few feet of yellowish and bluish clay 

 which is overlain by a considerable thickness of grit and conglomerate, such as is 

 described by Drake in his middle division of the Dockum beds. The conglomerate 

 and the resulting gravel, which in many places covers the surface, are generally very 

 easily distinguished from the grits and gravel derived from the overlying Tertiary 

 beds of the Plains by the large amount of browaiish, semiangular, indurated clay 

 as opposed to the dominant whitish, well-rounded quartzite of the upper beds. 

 The Triassic conglomerate is typically shown at Dickens, but can be traced from 

 that point as far as the Canadian River on the north and beyond Big Springs on 

 the south. At Dickens in Dickens County, Roaring Springs and Matador in 

 Motley County, and on the section through Quitaque to Tulia in Hall, Briscoe, 

 and Swisher Counties, this grit and conglomerate lies directly beneath the Tertiary 



