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that a small part of the rock is detrital and organic origin. Most of the rock, however, is fine and structureless, and if it were originally either detrital or organic, it must have been very different from the sand grains or the tests. Moreover, the content of silica in the novaculite is much greater than that of any tuff, which would seem to show that the rock is not directly of volcanic origin.
Significant features in the banded cherts-The banded cherts contain more undoubted elastic and organic material than the novaculites. The rock is divisible into alternating bands and laminae of different textures, which apparently represent the original stratification of the deposit. Some bands consist of fine structureless silica very similar to novaculite. Other bands contain a large percentage of sand and clay particles, and tests of Radiolaria and other organisms. These bands appear to grade into siliceous shales by a slight increase in the elastic components. The different colors in the bands may have been caused by variations in the organic and ferruginous material associated with the clays, and the bands may have been caused by periodic fluctuations in conditions of sedimentation in a region of relatively quiet water.
Conclusions-With. this inconclusive evidence, no definite interpretations can be made as to the origin of the novaculite. A relation between novaculite deposition and the secretion of silica by such organisms as Radiolaria, with volcanic activity, is possible but remains to be proved. To the writer the northwestward thinning of the novaculite members, their relation to the banded chert members, and the ripple-marked bedding surfaces suggest that the novaculite of the Caballos formation may have been laid down as a fine elastic sediment, rather than as a precipitate.
CARBONIFEROUS SYSTEM
PENNSYLVANIAN SERIES
The Pennsylvanian strata of the Marathon region were first studied by Baker and Bowman in 1915 and divided in ascending order into the Tesnus, Dimple, Haymond, and Gaptank formations. Later work has not modified this classification, though it has furnished more information on the ages of the different parts of the succession and has altered the interpretation of some of the exposures.
The formations are predominantly elastic, made up of sandstone and shale, with some beds of limestone and conglomerate. They reach a thickness of at least 12,000 feet in the southeastern part of the area but are much thinner to the northwest (pl. 8). The lowest formation, the Tesnus, is a mass of shales and fine sandstones 6,500 feet or more thick; the Dimple formation is a limestone as much as 1,000 feet thick; the Haymond formation, 3,000 feet thick, is again sandstone and shale, with a remarkable boulder bed in the upper part. The upper 1,800 feet of the succession, the Gaptank formation, is an alternation of limestones and conglomerates, with sandy and shaly beds. Marine fossils are found sparingly in the Dimple limestone and parts of the Haymond formation but are most abundant at numerous horizons in the Gaptank formation. Fossil plants, mostly abraded and poorly preserved, have been found at several places in the Tesnus and Haymond sandstones.
The folded and faulted lower formations crop out in rugged hills and ridges over wide tracts in the Marathon Basin. The higher parts of the succession are found only on the north side of the folded belt, where they have been partly overridden by thrust sheets that advanced from the south. The succeeding Permian strata to the north of them, in the Glass Mountains, are tilted away from the uplift and overlap across the folded and eroded edges of the Pennsylvanian.
TESNUS FORMATION
GENERAL FEATURES
The Tesnus formation was named by Baker and Bowman for exposures near Tesnus station, on the Southern Pacific Railroad east of Haymond, in the eastern part of the Marathon Basin. The Tesnus is the oldest Carboniferous formation in the Marathon region and is extensively exposed around the southeast, east, and northeast edges of the basin, where it flanks the central area of pre-Carboniferous rocks. It also crops out in narrow synclines between the anticlinoria of older strata. Because of the generally nonresistant character of its sandstones and shales, it occupies low places on the plains. In the northern part of the basin it is widely mantled by wash, but toward the south recent dissection has broken the old surface into an intricate topography of cockscomb sandstone ridges and shale valleys.
The Tesnus formation is a great mass of interbedded sandstone and shale, in thin and thick beds, nearly barren of fossils except for a few plant remains in the upper part. In most places it attains a thickness of several thousand feet, but its thickness is variable (pl. 8). In the northwestern part of the basin it is about 300 feet thick and is nearly all black shale, with few sandstone beds. In the southeastern part it exceeds 6,500 feet in thickness and is predominantly sandstone, with many arkose layers and several prominent massive layers of white quartzite. In this part of the area the basal part of the formation is predominantly shaly and "
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern Front Range of trans-Pecos Texas: Texas Univ. Bull. 1753, pp. 101-112, 1917. Preliminary descriptions of the formations are found in Review of the geology of Texas: Texas Univ. Bull. 44, 1916.
Baker, C. L., and Bowman, W. F., op. cit., p. 101.