544 C. N. GOULD CRYSTALLINE ROCKS OF THE PLAINS 



its summit. Others are much older, like Sierra Gran da, which, although 

 it now stands 2,500 feet above the plain, is so old that the crater has 

 been almost wholly destroyed by erosion. There are in this region great 

 numbers of surface lava flows, many of which now form high mesas, such 

 as Eaton, Johnson, Barillo, Mesa de Maya, Bartlett, and Chicorica. 



Still farther north, along the Eock}' Mountain front, is the Spanish 

 Peaks country, with its wonderful system of radiating basalt and mon- 

 zonite porphyry dikes; also in the region of Tertiary rhyolites near Castle 

 Eock, between Denver and Colorado Springs. 



OCCURRENCES IN THE ARKANSAS-TEXAS REGION 



In central Arkansas there are a number of exposures of intrusive 

 igneous rocks, which have penetrated the sedimentaries, sometimes the 

 Paleozoics of the Ouachita region and sometimes the Cretaceous of the 

 Coastal Plains. Crystalline rocks exposed south and southeast of Little 

 Eock and near Benton, some 20 miles southwest of Little Eock, are 

 chiefly syenite, known locally as gray granite. These rocks are of par- 

 ticular importance to the economic geologist on account of the fact that, 

 associated with them and with the adjacent Tertiary sediments, are the 

 most important known deposits of bauxite in America. In the Ouachita 

 Mountain region igneous rocks, chiefly syenite, are found in Magnet 

 Cove, about 12 miles east of Hot Springs. Scattering basic dikes of 

 later age than the syenites are also found from the syenite area near 

 Little Eock, westward to within about 30 miles of the Oklahoma-Arkan- 

 sas boundary. In Pike County there are four small exposures of peri- 

 dotite which form the source of the Arkansas diamonds.^ According to 

 Drake, it is generally believed that the syenites and Pike County peri- 

 dotites were intruded during middle Cretaceous times, probably during 

 the land interval separating the upper and lower Cretaceous, while the 

 basic dikes *of the central Ouachita region are probably Tertiary in age. 



Along the Balcones fault zone, in southwestern Texas, there are 

 numerous exposures of intrusive igneous rocks, consisting largely of 

 basalt and phonolite. These rocks were probably brought to the surface 

 in very late Cretaceous or early Eocene times. The chief exposures are 

 near Austin* and in L^valde and Kinney counties. In discussing the 

 occurrence of these intrusions in the Uvalde region, Vaughan says^ that 

 no surface flows of lava are known, but that the intrusions may be 

 divided into the following classes: (a) bosses, stocks, or necks; (h) 

 laccoliths; (c) laterally intruded sheets, and (d) dikes. Something like 

 65 separate exposures of igneous rocks may be noted on the Uvalde 

 quadrangle. 



