206 REPORT UNITED STATES GEOLOGICAL SURVEY. 



cur, also, a breccia, referred to Dolerite. From Middle Park southwest 

 but few volcanics are met with, until we repch the region of the Grand 

 Mesa.* In the Elk Mountains, volcanic rocks of the greatest interest 

 occur. Their unique character, as regards both constitution and asso- 

 ciation, claims for them the utmost attention. By far the largest area 

 is that of the Uncompahgre and the San Juan Mountains. t Stretching 

 for miles and miles, they there present a perfectly unbroken surface. 

 In the interminable varieties that have there been produced, and in the 

 harmony of detail arrangements, we have a series that may justly be 

 compared to a succession of sedimentary beds. 



In the Sawatch Eange, in the Sangre de Cristo, and in the Wet 

 Mountains, we have areas that, although isolated now, tend to show a 

 former connection over a large expanse of country. 



As typically isolated groups, isolated by virtue of their genesis, we 

 may regard the Spanish Peaks, La Plata Mountains, Mount Wilson, 

 Sierra Carriso, El Late Mountains, Sierra La Sal, and Sierra Abajo. 

 Although the latter two are not within the boundaries of Colorado, they 

 are very near, and are possessed of the same characteristics that else- 

 where must be regarded as typical for certain groups of Colorado. 



A large area of volcanic formations may be found in the southwestern 

 and southern portion of San Luis Valley. Though directly a continua- 

 tion of the same formations in the Southern San Juan Mountains, their 

 appearance in so great a mass in a valley is noticeable. 



Local outcrops of various volcanic formations, occurring in the form 

 of dikes and buttes, are frequently found throughout the State. Sev- 

 eral of the species of volcanic rocks participate in their formation, and 

 greatly dependent upon their composition will be the exterior appear- 

 ance they present. 



From this distribution it will be seen that the bulk of volcanic forma- 

 tions falls into Southwestern Colorado. Whereas the outlying groups 

 may be regarded as of but inferior importance, qualitatively, we there 

 have an enormous mass of the material. 



As might be expected, the distribution of volcanic formations within 

 certain areas is productive of definite orographic effects. Dependent, 

 to some extent, upon the lithological character of the rocks is the result 

 observed. Differences of such constitution must be sought for in the 

 physical rather than the chemical nature, and proportionate thereto are 

 the distinctive features presented. Wherever volcanic rocks cap either 

 prozoicf or sedimentary formations, their vertical limits can generally be 

 readily distinguished. As a rule, if superincumbent upon sedimentary 

 beds, they are a protection to them from erosive agents. When in- 

 trusive, either conformable with the strata or across them, their action 

 is frequently a preserving one, although not in so marked a manner as 

 in the former instance. In connection with the topographical features 

 as modified by the occurrence of volcanic rocks, we find those pertain- 

 ing to the drainage. IsTon-sedimentary and sedimentary formations are, 

 at all localities, found to have been subjected to by far more powerfully- 

 acting disturbing agents than the volcanics. While, therefore, we find 

 the latter more frequently in an undisturbed position, the former have 

 been removed in various directions from their original places of depo- 

 sition. This must, of necessity, produce a more schematical arrangement 

 of the drainage within volcanic areas. So far as observation goes, it 

 shows that in Colorado the volcanic drainage exhibits a more radial and 

 spiral type than can be found within the area of any other class of rocks. 



* Rep. U. S. Geol. Surv., 1874. t Reps. U. S. Geol. Surv., 1873; 1874; 1875. 



t See p. 104. 



