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Show 66 GEOLOGY OF THE HIGH PLATEAUS. intercalating with each other and presenting certain antitheses We have as the dominant group the true trachytesâ€"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 with Scrope's observations in the Auvergne. It has already been remarked that the volcanic 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 investigations 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 observers have done, those identical 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 advert for a moment to the relative age of the rhyolites. Instances occur where it is probable that some of the oldest liparitic outpours are considerably 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 effusion of some of the trachytes of the Sevier Valley. There are 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 correlated in any regular and progressive 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 constituents, specific gravity, texture, or other * The classification here adopted is fully set forth in the next chapter. |