854 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



REGION OF SPANISH PEAKS. 



One of the most highly favored regions in Colorado for the study of 

 dikes is that of the Spanish Peaks. Located east of the main passes 

 of the Sangre de Cristo Range they traverse the sedimentary formations. 

 North of West Spanish Peak two dikes extend for the distance of 8 to 

 10 miles unbrokenly through the Carboniferous strata. Erosion, which 

 may have required geological ages, has removed a sufficient amount of 

 sedimentary material to let the narrow walls project for several hundred 

 feet above the surrounding level. While the more easily disintegrated 

 material was carried away, the hard, unyielding rocks composing the 

 dikes have successfully resisted the repeated attacks. Preserving to a 

 great extent features that even comparatively slight erosive action 

 would efface, they have remained essentially intact. From the character 

 of the volcanic material composing them it is evident that mechanical 

 erosion will attack them but very slowly, unless preceded or accom- 

 panied by chemical decomposition. 



Dikes, projecting as walls, occupy various positions. They may be 

 found occurring on ridges and mountain-spurs, or they may extend for 

 long distances in a level region. In the former instance, it is their influ- 

 ence mainly, either directly or indirectly, that permitted the formation 

 of ridge or spur. By metamorphosis of the adjoining sedimentary 

 beds, these may have been rendered better able to resist erosion, or the 

 exposure of the dike-wall may prove to be a mechanical shelter for 

 other less resisting portions. When the dike-wall succumbs to decom- 

 position and erosion, it ends in the same manner as the walls above 

 described. Portions of it break down, destroying the continuity, until 

 finally rock-pillars alone remain to mark the former course. 



Throughout Colorado, dikes occur more or less frequently. They are 

 very uniform in their behavior regarding erosion, however, and as only 

 their wall-like appearance upon the surface here becomes of interest, 

 it is unnecessary to allude to more of them. What has been said above 

 will hold good for all occurrences of this nature. In geographical no- 

 menclature, their influence upon the character of scenery and landscape 

 has been acknowledged. Kames like " Fortification Creek" and "Mu- 

 ralla Peak" denote the existence of the typical wall-like projections of 

 volcanic rock. 



During 1875, Mr. Holmes had occasion to explore Southwestern Colo- 

 rado, From Navajo Creek, he publishes a very interesting sketch of a 

 double dike- wall.* The volcanic material there protrudes through Lower 

 Cretaceous strata. Subsequent erosion has removed the sedimentary 

 material surrounding it, so that at present the double wall extends 

 upward perfectly isolated. By the various remnants indicating the 

 trend of the dike, Mr. Holmes found its length to be more than a mile. 



Few occurrences, perhaps, can furnish us with data so reliable for 



*Eep. U. S. Geol. Surv. for 1875, p. 276. ' 



