A glance at the geological map of that part of the United States lying between the Mississippi River and the Rocky Mountains will reveal six regions of mountain uplift, namely, the Black Hills, the Ozarks, the Ouachitas, the Arbuckles, the Wichitas, and the Llano Mountains (sometimes called the Central Mineral Region) of Texas. The general structure of all these mountains is essentially the same, being in each case a truncated dome. Except in the case of the Ouachitas, the sedimentaries have been removed by erosion over the core of the uplift and the subjacent crystalline rocks have been exposed (see figure 1).
The greater part of the igneous rocks exposed in the cores of these various mountains is Precambrian in age, and the rocks vary considerably, both in composition and in texture. In the Black Hills the crystalline rock is largely gneiss, schist, and granite; in the Ozarks . . .