TRECAMBRIAN CKYSTALLINES 543 



The sedimentaries exposed, dipping quaquaversally along the flanks 

 of these various mountain ranges, vary in age from Cambrian to Ter- 

 tiary. In the Black Hills the sedimentary rocks include Cambrian, 

 Ordovician, Carboniferous, Triassic, Jurassic, and Cretaceous sedi- 

 mentaries; in the Ozarks, Ouachitas, and Arbuckles there are rocks of 

 each of the Paleozoic systems. In the AYichitas and Llanos, Cambrian, 

 Ordovician, and Carboniferous sediments are exposed. 



From time to time each of these mountains has been studied in much 

 detail by various geologists, and there has accumulated a large amount 

 of literature on each of the groups. It is not the intention at this time 

 to enter into the discussion of the crystalline rocks in the mountain 

 uplifts, nor, in fact, to do more than to briefly mention their occurrences. 



Crystalli]stes of later Age 

 general statemext 



Besides the Precambrian igneous rocks which occupy the cores of the 

 various mountain rang^es, there are on the plains several widely scattered 

 regions in w^hich occur crystalline rocks of Paleozoic or later age. The 

 greater part of these rocks are considered to be intrusive. They vary 

 greatly, both in comj^osition and in character. 



OCCURREXCES NEAR THE ROCKY MOUNTAINS 



Along the western margin of the Great Plains, and usually at no great 

 distance from the east flanks of. the Eocky Mountains, there are numer- 

 ous exposures of crystalline rocks. The Texas Geological Survey^ lists 

 16 ranges or groups of mountains in Trans-Pecos, Texas, south of the 

 line of the Texas and Pacific Eailroad, which contains igneous rocks, 

 the greater part of which are intrusives. Basalt dikes are known to 

 occur in southeastern New Mexico, where they cut the Tertiary; also 

 near Santa Rosa, in Guadaloupe County, east central N"ew Mexico, where 

 the volcanic rock penetrates Triassic Redbeds. 



The most notable example of volcanic action on the plains is the Raton 

 Mesa region of northeastern iSTew Mexico and southeastern Colorado, 

 extending from near the foothills of the Rockies eastward for a distance 

 of 100 miles, into extreme northwestern Oklahoma.^ There are in this 

 region scores, possibly hundreds, of volcanic cones. Some are compara- 

 tively young, like Capuline, which stands 1,300 feet high and is a perfect 

 type of cinder cone, with a crater a quarter of a mile in diameter near 



^ For I'eferences see page 55S. 



