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The upper limit or eastern border of the Basement sands, considered as a formation, runs nearly parallel with the basal or western line across Travis, Burnet, Lampasas, and Brown counties. From the northwest corner of Comanche County, which is also the northeast corner of Brown County, the eastern limit passes down the south side of the Leon River Valley, by Comanche, very nearly to the junction of the North and South forks of Leon River. It returns northwest along the northern side of the Leon, passes southwest of Desdimonia, Eastland County, west of Dublin, Erath County, and crosses the Houston and Texas Central Railway 4 miles west of De Leon. From Desdimonia this parting follows a northward course for 45 miles to the Brazos, near Powell's crossing. Twin Mountain, in northeastern Erath County, and Lipan and Buckner, in Hood County, are near this line. North of the Brazos the line of the upper contact turns west of north, passing near Hiner post-office, Parker County, and crosses the Texas and Pacific Railway nearly 4 miles east of Millsap. It continues 1½ miles west of Anthon, crossing the Whitt-Weatherford road 1½ miles from Whitt, Parker County; passing near Gibbtown, Jack County; Willow Point, Wise County, and 2 miles south of Bridgeport; crossing Trinity River at the mouth of Sandy Creek; Sandy Creek due west of Decatur; and the Fort Worth and Denver Railway 1 mile west of Alvord, Wise County; passing very near Denver, Montague, and Bonita, Montague County, and reaching the Red River Valley north of Belcher post-office, Cooke County.

This upper contact crosses the Fort Worth and Denver Railway 4 miles northwest of Decatur. At this point the basal Trinity and the upper or Paluxy sands blend and the summit of the Basement sands represents a higher geologic horizon than to the south. Opposite Decatur the parting between the two beds of sand is nearly midway between the base of the Fredericksburg division and the Trinity Paleozoic contact; it is believed to hold this relative position through the northern portion of Wise and Montague counties.

The eastern border of the sands forms a narrow belt down Red River from Belcher to the crossing of the river by the bridge of the Santa Fe Railway, and returns up the stream on the Territory side and thence north to a point northwest of Marietta 10 or 15 miles. From here their coastward border turns east, following the interior border of the Goodland limestone scarp, 6 miles northwest of Marietta, 2 miles north of Goodland, and crosses into Arkansas north of Cerro Gordo. The lower portion of Marshalls Bluff, Grayson County, is composed of the Basement sands surmounted at the top by the Goodland limestones.

When their vast extent is considered, the composition of the Basement beds is remarkably uniform, although there are a few exceptional