ART. XVIII -ON A PECULIAR TYPE OF ERUPTIVE MOUNTAINS IN 



COLORADO. 



By a. C. Peale. 



Since the summer of 1873, the LTuited States Geological Survey of the 

 Territories has been eugaged iu the survey of Colorado, the field-work 

 being finished during 1876. 



In reviewing the areas covered by the explorations of the Survey, 

 many additional facts in regard to the geologic structure of the Rocky 

 Mountains have been brought to light, none more interesting than the 

 one I propose to consider in this paper, viz, the occurrence iu Colorado 

 of eruptive mountains of a peculiar type. 



By eruptive mountains I mean those igneous and yet non-volcanic 

 mountain masses, due to sudden or catastrophic action as contradis- 

 tinguished from the uniform and long-continued action, the result of 

 contraction and lateral pressure, by which, iu general, mountain chains 

 and ranges have been produced. That they are the result, secondarilj^, 

 of the same causes I shall endeavor to point out in a subsequent 

 portion of this paper. There are a number of eruptive areas, which, 

 from a general resemblance in their rocks and in their mode of occur- 

 rence, should be classed together. They do not form mountain chains, 

 but are isolated masses, although, in a few cases, their proximity to 

 mountain ranges is at first the cause of some difficulty iu recognizing 

 them as distinct. 



The object of this paper is to present briefly and connectedly what 

 are conceived to be the principal features in regard to these areas, the 

 material being drawn from the results of the observations of my col- 

 leagues and myself, as detailed iu the various Reports of the Survey. 

 Future investigation in more detail than our limited time for observa- 

 tion allows, will doubtless add much to the knowledge of these isolated 

 eruptive mountains. 



In the first place, let me present briefly a description of them, com 

 menciug in the eastern part of Colorado. 



SPANISH PEAKS. 



The Spanish Peaks are two beautiful isolated mountains situated east 

 of the Sangre de Cristo Range, at the sources of the Apishpa and Ct.- 

 charas, two tributaries of the Arkansas River. They were studied by Di • 



