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Show THE OBSERVED ORDER OF SUCCESSION OF ERUPTIONS. 65 presently. The rhyolites as a group are decidedly }rounger than the trachytes. Wherever the two are found in contact the priority of the trachytes is, so far as observed, without an exception. Still, there is little question in my own mind that some of the more ancient rhyolites of the Tushar are older than many outbreaks of trachyte in other localities. Finally, the basalts are clearly the youngest of all eruptions If this stated the whole case, we should have the essence of Bichtho-fen's succession almost perfect. The qualification becomes manifest when we come to the study of the trachytic series. Blended with the heavy masses of trachyte, we find in all of the greater exposures rocks of a totally different character. These intercalary sheets belong to the sub-basic or nearly basic groups, and may be designated, according to their constitution [augitic trachyte], augitic andesite, or even dolerite. It will be seen at once that we have here a group of rocks united by certain common characteristics : First, the possession of notable quantities of augite, sufficient, in fact, to render that mineral a distinguishing compound; second, a similarity of habit and facies, which, though distinctly varied, yet vary within quite moderate limits. The habit and facies are markedly basaltic, being greater or less degrees of that characterization which is superlative in true basalt. The older varieties of these intercalary rocks sometimes carry a predominating amount of orthoclase, which marks them as augitic trachyte; sometimes predominant plagioclase, which relegates them to the augitic andesites. The later varieties exhibit those peculiar labradoritic feldspars in conspicuous, often " glassy," crystals, polarizing in gorgeous bands, with rare sanidin and copious augite included in a glass-bearing base. They are usually coarsely crystalline, and have the rough fracture of some typical trachytes, from which, however, they are separated both chemically and mineralogi-cally.* Such rocks would be designated by Zirkel augitic andesites, I presume, but it seems best (with the greatest deference to such an eminent author) to call them dolerites, and to restrict the designation augitic andesites to less basic* varieties. We have, then, in the age of trachytic eruptions, two series of lavas * No doubt it was such rocks to which Abich gave the name " trachydolerite." Deiters recognized in the Siebengebirge a regular transition from trachyte to dolerite. Zeitschr. d. d. Geol. Ges. 1861. 5 H P |
