368 HENRY S. WASHINGTON 



are found in many of the trachyandesites of the Auvergne 

 described by von Lasaulx' and Fouque.^ Many of the rocks of 

 the Euganean Hills also belong here. The series of trachydoler- 

 ites has been already described. The andesitic series would include 

 the dacites and andesites, with their definitions narrowed to 

 their original limits, 3 so as to include only the rocks whose feld- 

 spars are acid plagioclase — oligoclase to andesine. In the basaltic 

 series, carrying only labradorite and anorthite, we find the most 

 acid members well represented by the "pyroxene-andesites" of 

 Santorini, the greater part of whose feldspars have been shown 

 by the researches of Fouque'* to be basic plagioclase. These 

 have a high percentage of silica (65 to 69), and for convenience 

 such rocks might be called sa?itori?ntes. In the moderately acid 

 part of the series would be found rocks corresponding to many 

 of the labradorites of French petrographers, as well as many rocks 

 classed as basalts, but which are more acid than the normal, 

 such as those of the Lowenburg, Meissner, and the Vogelsberg. 

 These may or may not carry olivine, as is also the case with the 

 intermediate and basic trachydolerites and andesites. Toward 

 the basic end we would have the basalts proper, chiefly anor- 

 thite-bearing and with or without olivine ; while below these 

 would come such rocks as the limburgites and augitites. 



It will be seen from the above that the trachytes are rich in 

 alkalies and poor in lime ; the trachyandesites rather rich in 

 potash and also in soda and lime ; the trachydolerites rich in 

 potash and lime, but poor in soda ; the andesites rich in soda 

 and lime, but poor in potash and the basalts very rich in lime 

 and poor in alkalies, especially potash. 



A somewhat similar table is given by Fouque and Michel 

 Levy on page 155 of their Mineralogie Micrographique (Paris, 

 1879). The effusive rocks are divided according as they con- 



' Von Lasaulx, Neu. Jahrb., 1870, 187 1, 1872. 



^^ Fouque, Etude des Feldspaths, Paris, 1894, 254-270.^ 



3 Roth, Gesteinsanalysen, Beriin, i86i, p. XLV. Von Hauer und Stache, 

 Geologic Siebenbiirgeiis, 1863, 70, 79. 



* Fouque, Santorini et ses Eruptions, Paris, 1879; also Etude des Feldspaths, 

 317-320. The smaller groundmass microlites are more acid. 



