AN ANAL CITE-BEA RING CA MP TONITE 5 1 



the line bounding the two geographic regions, are the Red 

 Beds of Permo-Triassic age, with their associated gypsum 

 deposits. The relation of physiographic form to geologic 

 structure is most excellently exhibited. The hard granitic 

 rocks stand out as rugged peaks, with occasional gentle slopes 

 where the Carboniferous limestones are left on their flanks. The 

 Red Beds represent the softest rock of the region, and the 

 position of this outcrop is marked by a valley. 



Fig. i. — View of Camptonite butte, Las Vegas, New Mexico. 



The contrast between the Cordilleran region and the Great 

 Plains is both structural and physiographic. The latter region 

 is underlaid by the Dakota and Colorado formations, with occa- 

 sional remnants of the Montana and Laramie. Except near the 

 mountains, the Cretaceous formations are nearly horizontal, but 

 there they are upturned. Throughout Colorado and northern 

 New Mexico, the outcrop of the Dakota sandstone forms a series 

 of ridges locally known as "hog-backs." Las Vegas is situated 

 at the eastern base of one of them. 



About four miles slightly north of east of Las Vegas there is 

 a little butte of igneous rock of exceptional interest. The struc- 

 ture of the rock and its occurrence indicate that it was intruded 

 into the Cretaceous beds at a considerable depth, though it now 

 stands somewhat above its surroundings. 



