87
The western border region, in Texas, on the other hand, was completely submerged by the Cretaceous seas and is now reexposed by the denudation of the Cretaceous rocks which once overlay it.
Azoic Rocks. ¹
The oldest known rocks of the plexus are schists, granites, and crystalline limestones. Some of these may be Archean; others are Algonkian. They are certainly pre-Cambrian. Studies so far made have not been sufficient to differentiate the rocks or to determine finally their exact age and origin. They are now exposed in comparatively limited areas at a few widely separated intervals in Texas and Indian Territory, between which no relation has been established.
The old granitic outcrops adjacent to the borders of the Black and Grand prairies are seen in limited localities in Burnet County, Texas, the Wichita Mountains of Oklahoma, the Chickasaw Nation of Indian Territory, and the Ozark Country of Missouri.
In the drainage valleys of the Llano and Colorado, in Llano, Mason, Gillespie, and Burnet counties (the Burnet Country of the Central Province), erosion has stripped away layer after layer of the once overlying sediments and exposed older granitic rocks. These consist of protrusions into a series of pre-Cambrian metamorphic strata to which Walcott, who first pointed out their relations, gave the name of the Llano group, which is unconformably overlain by the Cambrian. The Llano group is composed of layers of schists, quartzite, and other rocks tilted almost vertically and overlain by subhorizontal strata of Cambrian sedimentary rocks. The Llano group is of Algonkian age and probably includes the oldest sedimentary rocks of the Texas region, which have been so altered and metamorphosed that remains of life have not as yet been found in them. The Burnet granites are clearly intruded into the Llano group and hence are of later age. They are also overlain by the Cambrian strata, and therefore are pre-Cambrian. There may be other granites of post-Cambrian age in the Burnet region, as has been asserted by Comstock, but their existence has not been demonstrated. The Llano schists and the Burnet granites are so intimately associated that for present purposes they will be considered collectively.
Southward, in Gillespie County, the granites are overlain by the Cretaceous rocks of the Edwards Plateau and are reexposed in small areas by the incisions of the Pedernales drainage. They are also penetrated by well drills beneath the surface of the Cretaceous limestones "
See Pl. LXVI.
² Some have treated this Burnet granite outcrop-the survival of an ancient configuration-as the remnant of primeval mountains which have stood since the earliest days of geologic time, above the seas which subsequently surrrounded them. This is a mistaken hypothesis, however, for these rocks were completely buried beneath the sediments of the Cretaceous ocean which covered them, and are now exposed only where drainage hag cut down to them through the overlying layers of later rocks.
Am. Tour. Sci., 3d series, Vol. XXVIII, 1884, p. 431.