ITALIAN PETROLOGICAL SKETCHES 367 



as the doctrine of evolution now underlies the classifications of 

 zoology and botany. 



It is through considerations of this sort that I have felt justi- 

 fied in proposing the new names in the preceding pages, and that 

 I sketched the following brief outline of a grouping of the feld- 

 spathic effusive rocks. In Table IV Brogger's idea is carried out, 

 and furthermore the two main divisions of the lime-soda feldspars 

 — the basic and the acid — are recognized. The reasons for the 

 rehabilitation of an earlier system of classification' need not be 

 gone into here, since the scheme represented is not proposed for 

 general adoption, but only for the purpose of showing the rela- 

 tionship to well-established types of such rocks as those of the 

 absarokite-banakite series and the ciminite-toscanite series. 



In Table IV, then, the rocks included are classified by their 

 dominant feldspars and by their silica content. In the first col- 

 umn we find the tracliytic series, which includes only the purely 

 or predominantly alkali feldspar rocks. These include the 

 rhyolites (both of the potash and soda series) and the quartz- 

 pantellerites ; the trachytes proper, such as those of Ischia, 

 Berkum, and Algersdorf, and the pantellerites ; while the most 

 basic members of this series are unknown at present, or perhaps 

 incapable of existence, their place being taken by certain leuci- 

 tites.^ The tracJiyandesitic series would embrace those containing 

 alkali feldspar and acid plagioclase (oligoclase-andesine) in 

 approximately equal amounts. Their most acid representatives 

 would include such rocks as the Icelandic rhyolites described 

 by Backstrom^ and the vulcanite of Hobbs;* those of medium 

 acidity include what are called oligoclase-trachytes, and are 

 typically represented by the domite^ of the Auvergne and many 

 trachytes of the Siebengebirge ; while their more basic members 



'Cf. J. Roth, Gesteinsanalysen, Berlin, 1861. 



' They would represent chemically and mineralogically in an effusive form the 

 basic augite-orthoclase shonkinite of Weed and Pirsson. (Bull. Geol. Soc, VI, 415, 

 1895.) 



3Backstr6m, Geol. Foren. Stockh. Forh., XIII, 637, 1891. 



^HoBBS, Bull. Geol. Soc, V, 598, and Zeit. d. d. geol. Ges., XLV, 578, 1893. 



5 This might be used as the name for the group of medium acidity. 



