272 REPORT UNITED STATES GEOLOGICAL SURVEY. 



may ratber suppose that certain portions are gradually cooled ofiF, are 

 again reheated to sufficiently high temperatures only to form compara- 

 tively low silicates, and are ejected as such. 



This view bears directly upon the point of extinction of volcanic 

 activity within certain circumscribed areas. Should the percentage of 

 silica in the material of any one reservoir become so high, that the tem- 

 perature and other solvent agents there existing are no longer suffi- 

 ciently great to produce fusion, then a cessation of eruptions must ensue. 

 In case no additional material is introduced within the range of the 

 reservoir, this cessation will remain permanent. As the result we will 

 find that all volcanoes or volcanic vents supplied therefrom must become 

 inactive. 



EECAPITULATION. 



So as to present in concise shape the main conclusions drawn from 

 the study of the eruptive rocks of Colorado, a short recapitulation is 

 here presented. 



I. The eruptive granites and trachorheites of Colorado are the direct 

 products of remelting of metamorphic rocks, mainly granites. 



II. With decreasing geological age of the trachorheites the percent- 

 age of silica increases. 



III. The basaltoids are the products of repeated fusion and cooling of 

 what originally were metamorphic rocks. 



IV The age of the oldest eruptives in Colorado, recognizable as such, 

 falls into the Post-Carboniferous era. 



Y. The age of the Elk Mountain granites, trachorheites and porphyri- 

 tic trachytes falls into a period subsequent to the close of the Cretaceous 

 formation. Eruptions probably continued throughout the first half of 

 the Eocene. 



VI. The age of the basaltoids in Colorado falls into the Tertiary 

 period, and in adjacent States and Territories it may be connected to 

 eruptions of the present epoch. 



VII. The main portion of the eruptions in Colorado was determined, 

 as to horizontal distribution, by the arrangement of maximum Creta- 

 ceous and, in part. Tertiary sedimentation. 



VIII. The period of greatest volcanic activity was prior to the main 

 elevation of the Eocky Mountains and the depression of adjoining re- 

 gions. 



IX. Volcanic eruptions, elevations, and depressions of these regions 

 stand in intimate, though inverted, connection to each other. 



X. The reservoirs, forming sources for Colorado, were of comparatively 

 shallow depth.* 



* Note. — I may here state that I am at present preparing a more exhaustive paper 

 on the origin of eruptives. This will discuss not only the erupted rocks of Colorado, 

 but of the entire West. — ^E. 



