116 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



In northern Brewster and southern Pecos Counties is a series of upper 

 Pennsylvanian rocks called Gaptank by Udden.^ 



"It consists at the base of conglomerates alternating with limestones, sand- 

 stones, and shales; in the middle part of shales interbedded with limestone; while 

 the upper portion is mainly composed of several masses of limestone separated by 

 shaly and sandy material. The thickness of the whole formation surpasses 1,500 

 feet. It probably corresponds to the highest portion of the Pennsylvanian of 

 central Texas — the Cisco formation." 



The Leonard formation is regarded by Udden^ as Permian, but seems 

 more properly to belong with the Pennsylvanian series. Udden describes 

 it as follows: 



"The upper part is composed of thinly laminated yellowish sandstone inter- 

 bedded with layers of gray limestone, yellow chert, and gray shales. 



"The lower part consists of heavy and thinly bedded gray limestone, in part 

 conglomeratic or containing pebbles of different size. At the base, shales and 

 soft sandstones are interbedded with a dark gray limestone. The thickness of 

 the entire series is nearly 1,800 feet. * * * 



"Farther to the east, near Word's ranch, the series is much more calcareous 

 in the upper and middle part; the thickness is more than 2,300 feet. Toward 

 the west of the first-mentioned section, the shales begin to predominate, while 

 the limestone is reduced in thickness. At the base of this formation is a con- 

 glomerate from 20 to 200 feet thick, and this unconformahly overlies the Pennsyl- 

 vanian." [Italics, Case. See also the description of the deposits in Trans- Pecos 

 Texas on a previous page.] 



To the north and west the stratigraphic equivalent, if not the continua- 

 tion of the Hueco, is the Kaibab limestone of the Aubrey formation in the 

 Grand Canyon section. A typical section from this region is given by 

 Darton.^ 



Feet 

 Limestone, variegated crimson and lemon-yellow, with nodules of chert and iron, to 



top of hills 50 



Sandstone, coarse, drab, sometimes pinkish, in places containing many quartz pebbles 



and imperfect vegetable impressions 2 



Massive cream-colored limestones with geodes containing calcite i6 



Chert 



Cherty limestone 2 



Blue limestone with Productus semireticulatus, etc., very abundant 4 



Cherty limestone, light blue, containing Productus semireticulatus, P. occidentalis, 



Spirigera suhiilita, Orthisina umbraculum, Rhynchonella uta, etc 175 



Shales, green, red, and white, and snowy gypsum 180 



Hard blue limestone containing crinoidal columns, spines of ArchcEOcidaris, Pro- 

 ductus, Spirigera, etc 100 



Soft lemon-yellow limestone with few Productus ivesi, etc 90 



Drab cross-bedded sandstones (Coconino). 



The connection between the two areas is shown by the occasional appear- 

 ance in uplifts of rocks of similar appearance and probably equivalent age. 



^ Udden, J. A., Review of the Geology of Texas, Bull. 44, University of Texas, p. 47. 

 2 Idem, p. 51. 



' Darton, N. H., A Reconnaissance of Parts of Northwestern New Mexico and Northern 

 Arizona, Bull. 435, U. S. Geological Survey, p. 29, 1910. 



