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has been called †"Rough Creek shale member" by Baker and Bowman, but that name is preoccupied in the Pennsylvanian of central Texas.

-Along the east side of the Marathon Basin, east of the novaculite ridges that bound the Dagger Flat anticlinorium on the southeast, the Tesnus and other strata of Carboniferous age are folded into broad open anticlines and synclines (sets. B-B', C-C', and D-D'-D", pl. 21). In this region the formation is about 6,500 feet thick. Only the upper part is exposed near Tesnus station, the type locality.

The basal shale member, the lower fourth of the formation, is clearly separable from the upper three- fourths, in which sandstone predominates. The basal member crops out in rolling hills along the flanks of the novaculite ridges and appears on aerial photographs as a dark-colored, featureless surface (pl. 18). It is mostly a soft greenish shale, with interbedded layers and lenses of pale-green argillaceous sandstone. There are also some thin beds of hard platy dark-blue or black shale. The shales are less competent than the sandstones above and the novaculite below and are irregularly folded and crumpled. In places they are sheared and macerated by faulting. The interbedded sandstones are most prominent near the top of the member, and it grades into the main body of the Tesnus formation above. In view of the lenticular character of the sandstone beds, it is doubtful whether the boundary as drawn is at the same level at all places.

The upper part of the formation is predominantly sandstone in massive ledges that stand in low parallel ridges, separated by shallow valleys carved from the interbedded shales. In aerial photographs the outcrops of the sandstones and shales stand out in a striking manner as alternating narrow light and dark bands, which clearly reveal the structure of the formation (pl. 18). The sandstone beds are mostly buff or green and are friable and somewhat arkosic. Some of the members are thick-bedded, and there are a few massive layers as much as 50 feet thick (pl. 9, A). Other sandstone beds, mostly fine-grained, are thinly laminated and flaggy, with numerous shale partings. Some of the layers near the middle of the upper member southwest of Haymond station are coarsely ripple-marked. Near Tesnus station sandstones in the upper part contain the pinnules of ferns. At several places there are layers of thin-bedded black, dull-lustered chert. The upper 300 or 400 feet of the Tesnus is predomi nantly black indurated splintery shale, with subordinate sandstone layers. The contact with the overlying Dimple formation is drawn at the lowest limestone layer interbedded in the shale.

The following section of the Tesnus formation, 6,520 feet in thickness, was measured between Peña Blanca Spring and the Haymond Mountains (sec. 6, pl. 8). The structure of the region is simple and is made plainly evident by excellent exposures, so that there is little chance for duplication by folding and faulting. The writer was aided in the instrument work on this section by A. G. Nance and John Bean.