from the Rosita Hills, Colorado. 467 



not be out of place to state, for the information of those who 

 have no grounds upon which to establish a personal opinion, 

 that the term u trachoreite " of Endlich has no petrographical 

 signification whatever. Almost all varieties of volcanic rocks 

 known in Colorado — a long series — may be found prominently 

 developed in areas mapped as " trachoreite " by Dr. Endlich. 



The Rosita Hills are remarkable, when compared with other 

 volcanic areas of the West, for the number of eruptions and the 

 variety of products in so limited a district. Volcanic activity 

 began, as indicated by the products seen, with an eruption of 

 an andesite carrying hornblende and biotite. The action was 

 explosive, for the product is wholly fragmental, consisting of 

 mud, tufa, and breccia, now exposed in very irregular relations. 

 The vent is not known, and probably lies under some 

 later flow. After erosion of the soft materials of the first 

 period came two massive andesite outbreaks, one more basic, 

 the other more acid, than the first. These overlap the earlier 

 breccia on the north and south respectively, and form prom- 

 inent cones and ridges. 



Succeeding these andesitic eruptions came a series of rhyo- 

 litic outbursts. The earlier ones were violently explosive as 

 shown by the agglomerate filling some of the vents, while the 

 later ones were more quiet, producing massive rocks, seen in 

 many short dikes cutting all the earlier andesites and the 

 rhyolitic agglomerate, and in thin sheets on all flanks of the 

 hills. Following the rhyolite came another andesitic magma, 

 welling out through long fissures which cut all earlier rocks. 

 Surface masses of the same rock are seen. It is a mica-augite 

 andesite, with some free silica. The last important eruption 

 produced a rock carrying a very slight excess of silica and 

 having the characteristic structure and mineral composition of 

 a trachyte. This magma came up through fissures some of 

 which are nearly three miles long, and clearly traverse every 

 rock that has been mentioned, excepting the dacite, which does 

 not lie in their course. The later rocks of this series are in 

 many places very fresh, while the older andesites are as a rule 

 far gone in decomposition. This general decay is mainly due 

 to thermal waters coursing through innumerable fissures. In 

 these decomposed areas are many small, metal-bearing mineral 

 veins. 



The area whose volcanic history has thus been outlined is 

 regarded by the writer as practically a volcano, whose phases 

 were of very different character at different times. Four out 

 of six important outbreaks produced massive rocks and but 

 two were of the explosive character more commonly seen in 

 true volcanoes. But the integral nature of the whole is 

 evident fr:>m the study of the mutual relationships of the 



