48

The marked changes in facies led in the earlier studies of the area to some confusion in the interpretation of the stratigraphy. In the preliminary announcement of their work, Baker and Bowman proposed the Caballos novaculite, with limits not clearly defined, and the overlying †Santiago chert, from 20 to 450 feet thick. The chert was said to have been named "from a locality at the east base of the Santiago Range, east of the range's summit." The †Santiago was rejected as a formation unit by them in 1917, because "later work by the senior author appears to indicate that they [Caballos and Santiago] are really one formation, two members of both the original Caballos and Santiago being included in the section at some localities." The writer's work has afforded complete confirmation of this later interpretation. (See foregoing table.)

The typical Caballos of 1916 (on Horse Mountain) apparently comprised the lower chert member, the lower novaculite member, the middle chert member, and the upper novaculite member of the present paper. On the other hand, the type †Santiago of 1916 (facies 2 of the northwestern Marathon Basin) consisted of the middle and upper chert members. As shown in the table, the upper novaculite member between the two chert members is thin or absent in this area and is not easily recognized. The type section of the †Santiago thus includes part of the type Caballos and suggests that the two formations should be included in one unit, as stated by Baker and Bowman in their final report. For this reason, in the present report only one formation, the Caballos novaculite, is recognized.

-In the northwesternmost exposures of the Caballos formation, in the Dugout Creek area (pl. 16), the formation is about 250 feet thick. Near the base is 10 or 20 feet of white vitreous novaculite, which is probably the equivalent of the lower novaculite member farther southeast. The remainder of the formation consists of layers of chert 1 to 8 inches thick, of dull to vitreous luster, banded by various dull colors, such as white, black, brown, green, and pale blue. Near the top are some dull-black cherts. The banded cherts in some exposures are crumpled, broken, and contorted by deformation. Most of the bedding surfaces of the cherts are smooth and regular, but some are wavy or hummocky. Many of the layers are separated by thin partings of hard greenish siliceous shale. At several localities thin lenticular layers of coarsely crystalline buff or gray siliceous limestone and dense laminated limestone are interbedded with the chert. Some of these contain angular chert fragments. A mile west of the Roberts ranch one of the limestone beds contains plentiful linguloid shells.

The following section, measured in the ridges in the southern part of the Payne Hills, 5 miles north-northeast of the Roberts ranch, shows the general character of the formation in this region (sec. 1, pl. 2; fig. 21).

Udden, J. A., Baker, C. L., and Böse, Emil, Review of the geology of Texas: Texas Univ. Bull. 44, 1st ed., p. 41, 1916.

Baker, C. L., and Bowman, W. F., op. cit., (Texas Univ. Bull. 1753) p. 100.

Idem. p. 94.