ENDLicH.] ACIDIC VOLCANIC ERUPTIVES — POKPH. TRACHYTE. 233 



Mount Eicharcl Ovvea.* Dr. Peale examined the region and has pub- 

 lished a dingrara thereof.t From this it appears that a series of radially 

 arranged dikes traverse Cretaceous shales, and by metamorphosing these 

 have given rise to the formation of a mountain. He does not regard 

 the rock composing the dikes as typically porphyritic. It is more com- 

 pact and of finer texture. Taking into consicleration the large number 

 of varieties presented by the tracLiyte and the comparatively intimate 

 association with other undisputable occurrences, I think that the occur- 

 rence may safely be regarded as belonging to this class. 



JSTorth and southwest of this mountain are extensive groups of por- 

 phyritic trachyte. They are more massive than is usually the case, and 

 comprise the region containing Mount Marcellina. Dikes traverse the 

 entire country there, connecting, in part, the detached areas of porphy- 

 ritic trachyte. Inasmuch as the mountains are not so completely iso- 

 lated here as is usually the case, these groups present a slight deviation 

 from the type accepted. 



SPANISH PEAKS. 



In 1869 Dr. Hayden first examined the Spanish Peaks. They are 

 located at the eastern entrance of La Veta Pass across the Sangre de 

 Cristo Mountains. Although within a short distance of this high moun- 

 tain range, the isolation of the Spanish Peaks is complete. They rise 

 from the adjoining low country to an elevation of 13,623 feet. Sur- 

 rounded on all sides by sedimentary beds, they have preserved the prin- 

 cipal feature distinguishing porphyritic trachyte eruptions — isolation. 

 Dr. Hayden characterizes them as " a gigantic dike."| This expresses 

 essentially their structure. During 1875 I visited them, and, in spite of 

 the advanced season, was able to collect some highly interesting data.§ 

 Structurally the two peaks differ. The eastern one may be regarded as 

 the main point of outflow for the volcanic material. While there the 

 main mass of the mountain is composed of porphyritic trachyte, the 

 western peak is chiefly built up of sedimentary rocks. There is a con- 

 nection between the central masses of the two, which appears in the 

 saddle dividing them. Through a main fissure, striking, probably, about 

 east 20° north, the trachyte ascended in a viscous or plastic condition. 

 Whatever may have produced the upheaval of the Carboniferous beds 

 through which the lava passed, resulted in parting many of them parallel 

 to their stratification. Into the wedge-shaped openings thus produced 

 the lava entered, forcing the strata to retain their distended position. 

 Simultaneous with or immediately following the initiatory upheaval 

 was a disruption of the rigid beds. This took place primarily in the 

 direction of east 20° north, the strike of the most extended line of erup- 

 tion. Issuing from that, and more particularly from its western termi- 

 nation, are a very large number of radial dikes. Some of these evidently 

 did not reach to the surface at first, but gradual denudation has brought 

 to light the material inclosed in the fissures. Standing on the summit 

 of West Spanish Peak the radiating dikes can readily be traced. They 

 form prominent, high walls, leading down from the mountain and some- 

 times extending into the plain below for a number of miles. Their dis- 

 tribution is a singularly regular one, and the constancy of each indi- 

 vidual dike, as regards its course, is surprising. From the present 



* Report of Reconnaissance in the Ute country, p. 40. 

 t Rep. U. S. Geol. Snrv., 1874, p. 165. 

 t Rep. U. S. Geol. Surv., reprint, 1867 to 1869, p. 153. 

 $ Rep. U. S. Geol. Surv., 1875, p. l-i8. 



