246 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



well be separated. By far the largest masses found were observed by 

 Mr. Marvine in doleritic breccia.* In the region of the Hot Springs, 

 Middle Park, Marvine observed heavy beds of doleritic breccia under- 

 lying strata belonging to the Post-Creta(}eous lignitic series. Their 

 maximum thickness is 800 to 900 feet. So far as could be determined, 

 this conglomerate must have been derived from former extensive de- 

 posits of dolerite and, in part, basalt. The rocks are characteristic, are 

 water-worn, and deposited analogous to true sedimentary beds. Their 

 relative position changes somewhat at different localities, but usually 

 they occupy a definite horizon. Most likely the material was ejected 

 prior to the eruption of the main mass of basalts, and an extensive 

 action of erosion has early produced the effects we now observe. This 

 is the chief occurrence of dolerite in Colorado, though I do not doubt 

 that upon caretul investigation certain groups heretofore referred to 

 basalt will be resolved into dolerite and diabase. 



BASALT. 



Basalt occurs at many localities throughout the State. It is found in 

 varying quantities and of different methods of occurrence. Similar to 

 trachyte, it takes a prominent part in shaping the exterior features of 

 the country. Analogous, also, to trachyte, we find its associations to 

 be constant within certain limits. 



Three types of eruption may be distinguished : 



Ilassive, 

 Anarhactic, and 

 Isolated. 



To the first we must count those basaltic areas that are in intimate 

 connection with the trachorheitic group, and those where enormous 

 masses of the lava have been poured out over sedimentary beds. 



The second group comprises occurrences that we observe most fre- 

 quently in the younger sedimentary formations. Fissures, mainly formed 

 in the strata, have been filled by basalt, and now are found as dikes. 



Isolated groups occur most frequently in the lower country, both 

 south and east of the main mountain-masses. They are thoroughly 

 characteristic in their features, as well as in the relations to surround- 

 ing rocks. 



I. — ^MASSIVE. 



San Luis Valley. — In the southwestern portion of San Luis Valley, 

 and extending from there westward, we find a very large area covered 

 by basaltic rocks. This area commences on the eastern slope of the San 

 Juan Mountains, reaching far down into the valley. Prominent in the 

 southern portion are two high peaks. Mount San Antonio and Ute Peak. 

 We find, by examining the western edge of the basalt area, that these rocks 

 rest directly and conformably upon the trachorheites composing the mount- 

 ains. From there the basaltic beds slope eastward, dipping toward the 

 valley. At all points the successive flows of lava can be readily traced. 

 They are perfectly distinct, and greatly aid in the recognition of the struct- 

 ure of the entire region. During 1875 I visited the region,t and made 

 an attempt at determining the points of outflow for the entire area. So 

 far as any definite opinion can be held, I incline to the view that Mount 

 San Antonio and Ute Peak indicate the former ijoints of outflow. A 

 study of these (topographically) isolated mountains, and the rocks com- 



* Kep. U. S. Geol. Surv., 1873, p. 156, &c. t Eep. U. S. Geol. Surv., 1875, p. 143. 



