THE OBSERVED ORDER OF SUCCESSION OF ERUPTIONS. 65 



presently. The rhyolites as a group are decidedly 3 T ounger 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 Richtho- 

 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 char- 

 acteristics : 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 predomi- 

 nating amount of orthoclase, which marks them as augitic trachyte ; some- 

 times predominant plagioclase, which relegates them to the augitic andesites. 

 The later varieties exhibit those peculiar labradoritic feldspars in conspicu- 

 ous, 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 tractates, 

 from which, however, they are separated both chemically and mineralogi- 

 cally.* Such rocks would be designated by Zirkel augitic andesites, I pre- 

 sume, but it seems best (with the greatest deference to such an eminent 

 author) to call them dolerites, and to restrict the designation augitic andes- 

 ites 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 recog- 

 nized in the Siebengebirge a regular transition from trachyte to dolerite. Zeitschr. d. d. Geol. Ges. 1861. 



5 H V 



