542 



C. X. GOULD CEYSTALLIXE EOCKS OF THE PLAINS 



truncated dome. Except in the case of the Ouachitas, the sedimentaries 

 have been removed by erosion over the core of the uplift and the sub- 

 jacent crystalline rocks have been ^sposed (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 consider- 

 ably, both in composition and in texture. In the Black Hills the crys- 



^^ PrE-CamBMAN CRVSTAUUtNE 



O 4- Post-Cam BRIAN Cr>''.3Ta Li-i •• a 



Figure 1. — Location of Occurrences of Crystalline Rocks on the Plains 



talline rock is largely gneiss, schist, and granite: in the Ozarks it con- 

 sists chiefly of granite and rhyolite, with intrusive dikes of diabase; in 

 the Arbuckles the igneous rock is chiefly coarse granite and gray and 

 reddish porphyry; in the Wichitas it is largely red and black granite, 

 gabbro, and porphyry, while in the Llano Mountains the igneous rock 

 consists largely of schist and gneiss, with intrusions of granite, gabbro, 

 and diabase. 



