240 REPORT UNITED STATES GEOLOGICAL SURVEY. 



observed elsewhere. Dikes have been formed, traversing the sediment- 

 ary strata, and composed of the same material that is shown in the main 

 mass. Newberry, in 1859, recognized the local eruptive character of the 

 group.* Although the Sierra Abajo is not within the limits of Colorado, 

 its relations with Colorado groups are so striking that a brief mention 

 has here been made. 



SIEKRA CARRISO. 



This, like the preceding, is not situated in Colorado, but in the ex- 

 treme northeastern corner of Arizona. It is similar, in every respect, to 

 the El Late Mountains. Mr. Holmes visited it during 1875, and pub- 

 lished an interesting account, t The highest points of this group rise to 

 an altitude of 9,000 feet, the most elevated of which has been named 

 Mount Pastora. Here, as in the other groups, porphyritic trachyte has 

 found its way through the superincumbent sedimentary strata, and has 

 spread out laterally, upon reaching such strata as would permit its ex- 

 pansion. It is highly probable that but a limited quantity of the lava 

 originally reached the surface, but that subsequent erosion removed the 

 Cretaceous beds, thus exposing the trachytes. Mr. Holmes has observed 

 a very curious occurrence in connection with the Carriso group. He 

 finds that the Cretaceous sandstones show a flexure which indicates the 

 primary formation of either an arch or a dome. Covering these strata 

 are layers or flows of trachyte that " are also flexed with the sandstones, 

 and appear as if they might at one time have formed part of the arch." 

 Two explanations present themselves for this i)henomenon : 



(1.) The eruption of volcanic material may have been spasmodic. 

 Long periods of time may have elapsed between the successive ejections 

 of lava. 



(2.) The overlying trachytes may form an, originally, intrusive sheet. 

 In this case the higher beds of sedimentary strata have been removed 

 by erosion. 



I am inclined to this latter view, from the fact that at most localities 

 the eruption of porphyritic trachyte appears to have progressed without 

 any appreciable intermission, and because there is ample evidence of 

 extensive erosion. 



Huge fragments of the sedimentary material have been carried upward, 

 together with the lava, and now present the characteristic metamor- 

 phoses observed at so many points. Portions thereof have undoubtedly 

 entered the trachyte, thus changing, to a certain degree, its composition. 



With the Sierra Carriso ends the discussion of such groups as have 

 been referred directly to the porphyritic trachytes. It remains to sum- 

 marize the results, and treat, more connectedly, of the various unique 

 features belonging to these groups. -^ 



PvESUME OF PORPHYEITIO TRACHYTE. 



I. — LOCATION. 



From the preceding pages it will be observed that the most noticeable 

 feature of the location of groups referable to porphyritic trachyte is 

 that of isolation. None of them are in direct connection Avith any mount- 

 ain range or groups. They stand in no intimate superficial relation with 

 other eruptions, whatever might be their associations near their sources. 

 Without exception, almost, they are surrounded by sedimentary beds. 



* Rep. Expl. Exp. J. N. Macomb, 1859, 187G, p. 100. 

 + Rep. U. S. Geoi. Surv., 1875, p. 274. 



