236 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



13,135 feet. Mr. Holmes basstndied this ^roup very carefully, and has 

 furnished a valuable report thereon.* To the eastward of Mount Hes- 

 perus is Mount Moss. This latter is composed entirely of porphyritic 

 trachyte, while the former is formed essentially by a succession of vol- 

 canic and sedimentary beds. Cretaceous shales, more especially, form 

 the main bulk of sedimentary material. After careful study, Mr. 

 Holmes prepared an ideal section,! which presents bis view of the 

 action that produced the formation of this mountain. Either the force 

 of the rising volcanic mass, or that initiating the eruption, has forced 

 the sedimentary strata upward, until they assumed the shape of a 

 dome. Vertical disruption and parting of the strata has resulted there- 

 from. Wherever the breakage was greatest, there the largest quantity 

 of volcanic material was ejected. It was forced vertically into the 

 fissures formed, and laterally into the interstratal spaces. Dikes and 

 interleaving volcanic rock show the evidence thereof. Eventually the 

 dome burst, and, through gradual denudation, the result, as observed 

 to-day, was achieved. Much of the sedimentary material must have 

 been broken, and could offer but slight resistance to decomposing and 

 eroding agents. 



This very complete intrusion of trachytic rock among sedimentary 

 beds has been productive of thorough metamorphosis. The network of 

 volcanic sheets envelops so large a proportion of sedimentary beds, 

 that these are altered to a high degree. Mr. Holmes cites one instance 

 where the Cretaceous shales are in contact with the trachyte of mount- 

 ains south of Mount Hesperus; he says: " The exact point of contact 

 cannot be determined, as the metamorphism has been so complete that 

 the shales seem to change gradually into trachyte." Instances of this 

 kind are not rare in the La Plata Mountains. They argue forcibly for 

 the acceptation of a very high degree of heat which the trachyte must 

 have possessed at the time of its ejection. As elsewhere, under similar 

 circumstances, the shales are changed into argillites; have become hard 

 and brittle. Toward the exterior portions of the mountain-group there 

 are numerous dikes; metamorphosis in their vicinity is thorough, but 

 as we recede from them we gradually find the unchanged shales 

 again. 



Regarding the trachyte, which composes the entire mass, it may be 

 stated that many varieties are found. As a type we may accept a gray 

 to greenish microcrystalline paste, with numerous opaque, white 

 crystals of oligoclase, minute crystals of black mica which occur spar- 

 ingly, and acicular crystals of black or dark-green hornblende. At 

 some localities the trachyte changes into a crystalline aggregate. Tais 

 is owing, so far as can be determined, to the fact that the volcanic 

 material has " absorbed" a large quantity of the rock through which it 

 passes. Although the same feature may be noticed elsewhere, it is 

 rarely so fully developed as here. I will have occasion to discuss this 

 more fully below. 



One of the most striking occurrences in the La Plata Mountains is 

 the existence of metalliferous veins. They occur with in the metamor- 

 phosed area, near the central portion of the mountains. Probably the 

 fissures containing them were formed at the same time with those now 

 filled by dikes and by the same force. Mr. Holmes observed no regu- 

 larity in their arrangement, however. He states that the veins frequently 

 leave the metamorphic and enter the unchanged sedimentary areas, 

 without any appreciable alteration of course or other characteristics. 



* Eep. U. S. Geol. Surv., 1875, p. 268. t Ibid., p.2 70. 



