THE BASIN PROVINCE 115 



Despite this uncertainty expressed by Girty, there seems to be a very 

 definite opinion in the minds of most workers in this region that the fauna 

 and horizon equivalent to the Hueconian west of the Rocky Mountain barrier 

 is GscheHan in age. 



Beede' has expressed the opinion that the Delaware Hmestone is upper 

 Pennsylvanian and that the Hueco is Mississippian in large part. This 

 opinion is frequently expressed by Girty himself, in other papers. 



Though the Hueco and its equivalents occupy a position considerably 

 below the top of the Pennsylvanian, they afford a very convenient base 

 from which to reckon the changes which occur in upper Pennsylvanian and 

 Permo-Carboniferous time, and for this reason is traced in considerable detail 

 in the following pages. Except in the Guadalupe Mountains and the 

 regions far north in the United States, western and northwestern British 

 Columbia, and in Alaska, this limestone horizon is followed by shale and 

 sandstone beds which lead up to red beds or their chronological or conditional 

 equivalents. In the Guadalupe Mountains such a transition is lacking, 

 the Delaware limestone following the Hueco limestone directly; in the other 

 regions cited, the red beds and the transition beds, if ever present, have 

 been wholly or in part removed by erosion. 



(a) Conditions i?i Texas. — Udden states of the Hueco limestone formation 

 that it — 



"consists almost entirely of a gray, hard, thick to thin-bedded limestone, which 

 contains very little magnesia, or none at all. At the base of this limestone 

 generally occur yellow to brown and purple sandstones, and conglomerates, as 

 well as some gray and yellowish shales. The entire formation is at least 5,000 

 feet thick." 2 



It occurs pretty widely through Trans-Pecos, Texas, outcropping in the 

 various uplifts; the southern limit is unknown. South and southwest of 

 the Guadalupe Mountains in the Shafter region, Presidio County, the 

 Cieneguita beds consist of — 



"dark to black shales alternating with dark limestones, conglomerates, and heavy 

 lenticular masses of a clastic rock composed of siliceous fragments cemented by 

 calcareous clay (mortar rock). The shale predominates. Locally, some layers 

 of black chert occur. This whole series of rocks is at least i ,000 feet thick." * * * 



"The Alta beds, which rest on the Cieneguita beds, show a thickness of about 

 3,500 feet. They consist of some dark shales below and some yellow sands above. 



"The dark shales consist of sharply bedded layers of silt, clay, and some sand, 

 with layers of coarser and more purely sandy material. The thickness of this 

 series is approximately 2,000 feet. 



"The yellow sand consists of a soft, occasionally almost crumbling, bluish- 

 gray sandstone of fine texture. It is a coarse silt of well-assorted quartz grains. 

 The thickness of this series is about 1,500 feet." 



' Beede, J. W., The Correlation of the Guadalupian and Kansas Sections, Amer. Jour. Sci., 



vol. XXX, p. 131, 1910. See his figure reproduced as figure 2 of this paper. 

 ^ Udden, J. A., Review of the Geology of Texas, Bull. 44, University of Texas, p. 48, 1916. 



