ENDLicH.l EEUPTED EOCKS IN NORTH AMERICA AND COLORADO. 205 



serve the existence of the youngest volcanic activity, reaching to the 

 present day. The Western section of the North American continent 

 may consequently be subdivided into two groups, into the ancient and 

 recent volcanic. Of these, the former may be regarded as existing in 

 the Eocky Mountains proper, while the second comprises those portions 

 contained in the ranges near the Pacific Coast. This latter series is the 

 one that extends directly southward, and finds its first great develop- 

 ment in the volcanoes of Mexico and Central America. 



It is not the object of this paper to discuss extensively any of the 

 eruptives besides those of Colorado ; therefore these brief indications as 

 to distribution must suffice. We have in Colorado erupted rocks that 

 belong to the first group. Perhaps no section of the Western country 

 is so well adapted to the study of volcanics as Colorado. This is owing 

 to the great variety of rocks exhibited and the large expanses they 

 cover. During the four years occupied by the survey of this State, all 

 occurrences there have been thoroughly studied, and we are enabled 

 now to present a more complete synopsis than probably can be furnished 

 of any other Territory or State. In the subjoined pages, therefore, the 

 erupted rocks of Colorado are the main subject of discussion, and only 

 such cases as are necessary for comparison will be introduced. 



OCCURRENCE OF ERUPTED ROCKS IN COLORADO. 



Throughout the State we find volcanic rocks distributed. In part 

 they are continued into adjacent Territories. Taking a comprehensive 

 view of all the groups, both lithologically and topographically, it ap- 

 pears that Colorado contains occurrences which, all taken together, form 

 a closed, a compact series. Whichever volcanic area may extend be- 

 yond the limits of the State, or whichever group of characteristic forma- 

 tions may be continued into either New Mexico or Utah, we neverthe- 

 less find its most important portion in Colorado. Very justly, therefore, 

 the volcanic groups of the State may be treated as one, representing 

 typically an important era in geological history. They may be regarded 

 as chapters of the book which is commenced and finished within the 

 area assigned to the State. 



Distribution. — During the progress of the Survey, volcanic rocks were 

 almost everywhere encountered. Extensive areas were found, where all 

 other formations were successfully hidden bereath the superimposed 

 masses of volcanic material. Again, the latter had formerly covered 

 much that erosion gradually brought to light once more, and we now 

 see it in isolated particles. Groups, whose isolation and small hori- 

 zontal extent is a leading characteristic, occupy prominent positions in 

 the landscape of Colorado. 



We find the volcanic exposures along the various mountain-ranges of 

 the State more connected than elsewhere. In the Front Eange, in th& 

 vicinity of Pike's Peak,* we first meet with extensive volcanic areas. 

 Occupying a position which imposes them upon the granite of that 

 range, they form for some distance its most elevated points. Proceed- 

 ing westward, we are accompanied by volcanics into the southern end 

 of South Park.f Although not so continuous as southwest of Pike's 

 Peak, the areas are well defined and sufficiently j)rominent to be of im- 

 portance orographically. 



Farther north, in Middle Park, we again find isolated groups of vol- 

 canic rocks.f In the vicinity of the Hot. Springs basaltic outcrops oc- 



* Rep. U. S. Geol. Surv., 1873, p. :U8. tRep. U. S. Geol. Sarv., 1873, p. 219. 



t Rep. U. S. Geol. Sarv., 1873, p. 162. 



