ENDLicH.] ACIDIC VOLCANIC EEUPTIVES PORPH. TRACHYTE. 235 



bas been fraugbt with the absorbiug interest presented by the Spanish 

 Peaks. l!fot only have we before us a volcanic rock of most peculiar 

 type, but we have an opportunity, rarely afforded, to study directly the 

 multiplicity of effects produced by volcanic action. 



Petrographically, the trachytes of the Spanish Peaks belong to the 

 great class that is so well chiiracterized. A microcrystalline paste con- 

 tains minute crystals of black mica. The color of the paste varies, but 

 gray and greenish tints predominate. Opaque crystals of white oligo- 

 clase are dispersed thronghout the entire mass. In sharp contrast to 

 them are black or dark-green acicular crystals of hornblende. Minute 

 grains of quartz occur sparingly, and probably have their origin in the 

 sandstones through which the lava passed. It remains to establish the 

 relations of all these rocks by the aid of the microscope more definitely, 

 but I am persuaded that the correlations thus far made will be correct 

 in the main. 



HUERFANO REGION. 



The Huerfano region stands in close relation to the Spanish Peaks. 

 As belonging to one group, I regard the Sheep Mountains and Muralla 

 Peak, south of Huerfano River.* They are composed of porphyritic 

 trachyte. Yeta Peak is the most southerly one. Drawing a line along 

 its crest, it will strike the two hills farther north in a direction of north 

 21° west. Although there is no apparent connection between them 

 above ground, they are so completely alike in every respect, that I as- 

 sume a continuity of the volcanic material lower down. Muralla Peak 

 shows a number of dikes radiating from its centre. They have given 

 rise to the formation of sharp ridges and are prominent features in the 

 landscape. While the trachyte of Yeta Peak is one of a typical char- 

 acter, that of Muralla Peak verges closely upon basalt. But few segre- 

 gated minerals are found in it, and those only of very small size. From 

 the intimate association, however, and the relative position and appear- 

 ance of this rock, I refer it to the same group. 



Badito Peak, at the southern end of the Greenhorn Mountains, be- 

 longs to the same class. In every particular is it conformable with the 

 typical occurrences. A number of dikes, one of them reaching a 

 length of 13 miles, occur independently in the same region. Porphyri- 

 tic trachyte composes them, and some of them bear evidence of not 

 having reached the surface at the time of eruption. 



That entire section of country along the Huerfano and Cucharas Riv- 

 ers has been one of maximum disturbance during the Post-Cretaceous 

 period. It is highly probable that the time of eruption will fall within 

 the Tertiary period. Enormous forces must have been brought to bear 

 in order to produce the extensive rents that we observe to-day. Un- 

 doubtedly many of the flexures of sedimentary strata in that region are 

 due to volcanic activity, the evidences of which are not brought to the 

 surface by denudation. It seems as if a large portion of the strata 

 lying far below must be completely reticulated by the action of anarhac- 

 tic force. That but a small portion, comparatively speaking, of the 

 power employed could make itself felt upon the surface, may safely 

 be assumed. 



LA PLATA MOUNTAINS. 



The La Plata Mountains are located in the southwestern portion of 

 Colorado, at the headwaters of Rio Mancos and Rio La Plata. Mount 

 Hesperus is the highest peak of the group, rising to an elevation of 



*Rep. U. S. Geol. Surv., 1875, p. 133. 



