66 GEOLOGY OF THE HIGH PLATEAUS. 



intercalating with each other and presenting certain antitheses We have 

 as the dominant group the true trachytes — rocks having the characteristics 

 of the sub-acid class, and augitic rocks with the characteristics of the sub- 

 basic class.* And it is interesting to compare this association witli Scrope's 

 observations in the Auvergne. It has already been remarked that the vol- 

 canic phenomena of the High Plateaus reveal a striking similarity to those 

 of Central France, though upon a much grander scale. Scrope frequently 

 alludes to the general impression prevalent long before he made his investi- 

 gations in that region, and held by many at that time, that the basalts were 

 younger than trachytes, and he frequently contests the correctness of that 

 opinion. Time and again he cites instances where he finds basalts lying 

 beneath trachytes as proof that the rule is by no means invariable. It 

 would be most interesting to know whether he has not included among his 

 " basalts," as scores of other most careful obsei'vers have done, those iden- 

 tical rocks which have here been described as augitic trachytes, andesites, 

 and dolerites, and which a more rigorous classification would separate from 

 the basalts. 



Leaving here these groups to return to them presently, we may ad- 

 vert for a moment to the relative age of the rhyolites. Instances occur 

 where it is probable that some of the oldest liparitic outpours are consider- 

 ably more ancient than some of the youngest trachytes. No infraposition 

 of rhyolite to trachyte has been observed in situ, but indirect reasoning 

 leads to the conclusion that the central rhyolitic masses of the Tushar were 

 erupted long before the efi"usion of some of the trachytes of the Sevier 

 Valley. There ai"e many instances in the Markagunt of rhyolite overlying 

 trachyte, and the more recent age of the former as a group is perfectly 

 apparent and incontestible. Lastly, the true basalts everywhere reveal 

 their greater recency than all other rocks. 



It now becomes of interest to inquire whether this sequence is cor- 

 related in any regular and progi'essive manner with the physical properties 

 or constitution of the rocks themselves ; whether there is with the progress 

 of the volcanic cycle any regular or systematic method of variation in the 

 chemical constitution, mineral constittients, specific gravity, texture, or other 



* The classification here adopted is fully set forth in the next chapter. 



