LACCOLITHS IN SOUTHEASTERN COLORADO.^ 



The western part of Colorado is mountainous ; the eastern 

 belongs to the Great Plains. The plains are in part smooth, as 

 the name implies, and in part broken by canyons and diversified 

 by valleys, cliffs, and terraces. Near the southeast corner of the 

 state is a broad upland plain, bounded on the north, south, and 

 west by bluffs that overlook lowlands with diversified relief. 

 The plain slopes gently toward the east and is furrowed here and 

 there by streams flowing in the same direction. Its determining 

 formation is an alluvial deposit of sand and gravel, believed to be 

 of Neocene Age. This rests on an eroded surface of Cretaceous 

 and Juratrias rocks, and these rocks are exposed in the surround- 

 ing lowlands as well as in the channels of the dissecting streams. 



Previous to the modern dissection the alluvial plain must 

 have been remarkably even, but a few knobs of resistant rock 

 projected above it, and one of these now stands so high as to 

 constitute a conspicuous landmark. Twin Butte, or Two Buttes 

 as it is sometimes less aptly called, is conical in its general form 

 but has a double summit. Past its southern base flows Two 

 Butte Creek, and the rock exposures are continuous from the 

 butte to the creek. The crest of the butte is 350 feet above the 

 plain and 600 feet above the creek. It stands in west longitude 

 102° 33' and north latitude 37° 39'. 



Visiting the locality in September 1895, ^ found the butte to 

 be capped by a block of sandstone which had acquired excep- 

 tional hardness through association with a local occurrence of 

 igneous rocks ; and a hasty examination of neighboring expo- 

 sures discovered such an arching of the Mesozoic strata as to 



' The observations here communicated were made in connection with field work 

 of the U. S. Geological Survey, and are published by permission of the director. 

 Proof of this article has not been read by the author. — Ed. 



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