200 REPORT UNITED STATES GEOLOGICAL SURVEY. 



crust. Evidence shows that during every one of the accepted geological 

 periods rocks have been ejected through the solid strata, and having 

 found their way to the surface, have been productive of changes cbarac- 

 teristic in every respect. Geognostically and lithologically such forma- 

 tions can readily be distinguished. Their mode of occurrence, the dis- 

 tinctive features of their form and distribution are unmistakable, should 

 even the lithological or mineralogical constitution afford no direct proof 

 of their origin. In intimate connection with the genesis and structural 

 peculiarities of volcanic masses is the eftect they produce upon the 

 topography or orography of any given region. Their presence may be 

 surmised from the configuration of the country, from the detail features 

 of mountains, and other exterior forms. Three groups, mainly, may be 

 distinguished from the standpoint of a topographer : those forming 

 isolated peaks or groups of peaks; those forming mountain chains and 

 ranges ; and the most complete, those forming extensive highland 

 plateaus. Each of these is referable, within certain limits, to certain 

 classes of eruptive rocks, and thus the exterior appearance will furnish 

 a suggestion as K) the material composing any one of the groui)S men- 

 tioned. Great variations naturally occur from any one rule that might 

 be laid down, and a systematic arrangement based upon distinctions of 

 this kind must necessarily, therefore, be but imperfect. 



Not only for the geologist and topographer are these erupted masses 

 of direct importance, but also for the miner and agriculturist. Discov- 

 eries and observations have developed a knowledge of certain facts in 

 connection with the occurrence of masses belonging to this group, that 

 demonstrate the value of acquaintance with them for one interested in 

 economic featuries. Mines of great magnitude and wealth have been 

 found within their limits, both in Europe and on this continent. Al- 

 though geological science is but young as yet, the knowledge gained 

 has been aptly ai)plied, and we have learned to utilize even the small 

 amount we posess to the greatest possible advantage. It is evident, 

 therefore, that the more thorough recognition of every feature connected 

 with such a class of rocks will furnish additional information that may 

 directly be applied in the process of supplying our wants. Thus the 

 agriculturist has learned that — conditions of climate and elevation be- 

 ing not unfavorable — he can successfully draw revenue from soil result- 

 ing from certain rocks, while others will claim by far more of his atten- 

 tion and labor. 



In the subjoined pages all questions pertaining to the origin, dis- 

 tribution, general and special features of the volcanic formations of 

 Colorado shall be considered. Wherever requisite, for the purpose of 

 clearer understanding, comparisons have been instituted with the anal- 

 ogous or identical formations of other regions, which in part, at least, 

 have been more thoroughly studied than those to which this paper is 

 devoted. Colorado offers more favorable points of discussion, perhaps, 

 than any other region of the United States, and as four years' work 

 there has brought me into constant contact with the consideration of 

 volcanic groups of that State, I shall endeavor to present as complete 

 a synopsis of their entire character as my investigations enable me to 

 prepare. The survey of the State is finished, the distribution of geo- 

 logical formations throughout its entire area is known, and this treatise 

 is, therefore, presented as a review of one of its most interesting classes 

 of rocks. 



Using the more comprehensive term, erupted masses, it becomes a 

 matter of some difficulty to draw the line of distinction. Where origi- 

 nal metamorphic rocks end and those brought to the surface by eruption 



