97

The entire series of the Carboniferous and Permo-Carboniferous beds of this section, from the underlying Silurian to the overlying Permian, was divided by Tarr into six formations, called "divisions," based upon lithologic and stratigraphic characters, as follows:

Tarr first observed the broad fact that the rocks comprising these divisions were groupable into three great lithologic categories, a, lower group of more massive limestones, a middle group of sand and shale, sandstones, conglomerates, etc., and an upper group of limestones.

Basal limestone

.-This division, which Tarr recognized but did not name, consists for the most part of massive, hard limestone, but at the top there is usually a bed of black clay shale. This rests upon the Ordovician rocks, forming a narrow belt on the north and northeast of the Burnet area.

Tarr gave no name to this lower formation, but it may probably be the same as that of the Smithwick Country which had been previously called the Marble Falls limestone and which was later called by Cummins the "Bend division." Tarr referred this lower limestone group to the "sub-Carboniferous" period upon the evidence of a fossil goniatite determined by Prof. Alpheus Hyatt. He stated that it consisted almost entirely of crystalline limestone with beds of calcareous shales occurring beneath the Upper Carboniferous beds and resting uncomformably upon the older Silurian rocks. In the later Texas reports the thickness of this limestone is placed at 360 feet.

The strata of the overlying divisions are said to be unconformable with this, but conformable to one another, which conditions are shown in the map and sections accompanying Drake's report. According to the last-mentioned authority, the strike of the basal limestone is nearly at right angles to that of the overlying beds. More field work is necessary, however, to fully establish this conclusion.

The Richland, Milburn, Brownwood, Waldrip, and Coleman divisions of Tarr (later called the Strawn, Canyon, Cisco, and Albany divisions by Cummins) constitute the plexus overlying the Marble Falls limestones, "

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The thicknesses cited are those given by Drake: Report on the Colorado coal field of Texas, by N. F. Drake and R. A. Thompson, Austin, Texas, September, 1893.

A preliminary report on the coal fields of the Colorado River, by Ralph S. Tarr: First Ann. Rept. Geol. Survey of Texas, 1889, Austin, 1890, pp. 201-233.

See Geological story of the Colorado River: Am. Geologist, May, 1889.

First Ann. Rent. Geol. Survey of Texas, 1890.

Second Ann. Rent. Geol. Survey of Texas, Austin, 1891, table on page 361.

Second and Third Ann. Rents. Geol. Survey of Texas, 1891.