89

 

been frequently considered an Archean nucleus, have been shown to be of Algonkian age. It has been generally thought that the Ozark uplift represented the survival of an ancient upland, but Marbut, Davis, Griswold, Keyes, and others who have recently given the region special study from both the geographic and the geologic standpoint, agree in regarding the uplift as it now stands as a very modern feature of relief, not earlier than middle or late Tertiary.

Paleozoic Sedimentary Rocks.

The sedimentary Paleozoic rocks of the Texas region, constituting the floor of the Cretaceous formations, belong to five great systems, the Cambrian, Ordovician or Lower Silurian, Upper or true Silurian, Carboniferous, and Permo-Triassic. The Devonian may also have inconspicuous representation in the complex of the Ouachita Mountains of Indian Territory, but need not be considered at present.

CAMBRIAN ROCKS.

The oldest of these systems of formations, the Cambrian, and the overlying Ordovician, which, so far as known, succeed each other

without break, may, for the purposes of this paper, be considered as a unit. These accompany and overlap the borders of the fundamental granite outcrops of the limited Burnet locality. The lowest and oldest of these is chiefly a sandstone of Middle Cambrian age with alternations of limestones. The second and overlying member, the Ordovician, is composed of firm limestone of close texture and great hardness.

The Cambrian formations, which are the oldest known fossiliferous sedimentary rocks, occur as the summits of buttes within the general area of the Azoic rocks of the Burnet basin and as a ring of outcrops "

---

Charles R. Keyes, in Science, April 29, 1898, pp. 588-589.