400 PROCEEDINGS OF THE CALIFORNIA MEETING 



The true nature of these rocks was not recognized in the field, and, indeed, the 

 base of the series was not determined for many localities until the specimens 

 were microscopically studied." 



Flow structure seems to be especially characteristic of portions of the Potosi 

 Rhyolite Series, and in this case the inclosed fragments are partly andesitic, 

 but mainly rhyolite, similar to that of the massive flow. Of this Cross says : 

 "The fragmeutal character is most evident near the bottom of the band, and in 

 several sections a massive flow with many inclusions — a flow-breccia — follows 

 without any clearly defined line of separation ; in fact, some specimens col- 

 lected to represent gravelly tuff were found on microscopical examination to 

 be flow-breccia." 



Other flow-breccias have later been described by Cross as occurring in the 

 San Juan Mountains. Some of these contain fragments like the fluidal ma- 

 trix; others like that of the Intermediate Series of the Telluride (luadrangle, 

 fragments different from such matrix. This is notably true of a flow-breccia 

 in the Silverton quadrangle, to which the name Eureka rhyolite has been 

 given. Of this rock Cross saj's :" "The second member of the Silverton Series 

 is a rock belonging to the most siliceous of the magmas, which were erupted 

 during this epoch of the San Juan volcanic history. It is so characterized by 

 small included fragments of andesites, of rocks very similar to the rhyolite 

 itself, or occasionally of granite, schist, etcetera, and has so commonly a flow 

 structure that much of the rock may be called a flow-breccia. . . . Nor- 

 mally it is a grayish rock exhibiting many small angular inclusions, averag- 

 ing much less than half an inch in diameter, of dark, fine-grained andesites or 

 of reddish or grayish rhyolite, and has a prominent fluidal texture in the 

 dense felsitic ground-mass which holds the fragments." 



A flow-breccia has also been described by Crawford^" as occurring on Brittle 

 Silver Mountain, in the Monarch-Tomichi district. Here, again, we have a 

 flow-breccia that contains fragments of porphyry not identical with the ma- 

 terial composing the lava flow in which they are inclosed. 



A flow-breccia, then, according to the definition established by Cross, may be 

 considered to be a rock having an outward resemblance to a true breccia, of 

 which the cementing material is a fluidal massive lava and the inclosed frag- 

 ments similar to or different from the fluidal matrix. 



Such flow-breccias are, according to the observations of the writer, of very 

 great frequency, not only in the San Juan Mountains, but in the outer lying 

 extension of the San Juan volcanic series to the east and north and also in 

 other parts of Colorado. 



In Saguache County, for insti^nce, in the Bonanza district, is a very marked 

 case of a latite surface flow stretching for a distance of at least 12 miles, in 

 every part of which fragments of inclosed andesite may be observed, in some 

 parts so thick as to closely resemble a true breccia. In the Platora-Summit- 

 ville district, in Conejos County, some 50 or 60 miles southeast of the San 

 Juan Mountains, occurs a flow-breccia very similar to some of those of the 

 Potosi Series; and in another part of this same district was observed mon- 

 zonite porphyry with inclosed fragments of monzonite. 



Again, in the Bonanza district, there occurs a rock closely resembling a 



» U. S. Geological Survey Folio No. 120, 1905, p. 7. 



10 R. D. Crawford : Colo. Geological Survey Bull. No. 4, 1913, p. 176. 



