THE VOLCANIC ROCKS OF THE ANDES. 1 75 



igneous rocks already cited. There is nearly the same relation 

 between the alkalies and alumina, the soda being still more in 

 excess of potash, and both increasing gradually toward the more 

 silicious end of the series. The alumina maintains a high posi- 

 tion, gradually decreasing. The lime, iron-oxide and magnesia 

 decrease rapidly from the less silicious to the more silicious end 

 of the series, and lie somewhat close together. In one instance 

 there is a marked drop in the magnesia. 



In Table II are given the analyses of lavas from the Andes 

 south of Colombia, with one exception, as they have been 

 recorded in Roth's tables of rock analyses. They present a some- 

 what wider range of silica percentages, from 52.02 to 73.61, but 

 whether they have all been made from unaltered rocks is not 

 known to the writer. An analyis of " obsidian " from Colombia 

 is omitted, since it is extremely crude, giving 75 per cent, of silica 

 and 3 per cent, of magnesia, with no lime. Diagram 2 expresses 

 the molecular variations among the rocks included in this group- 

 ing. They have the same general nature as those just described, 

 especially as to the alkalies and alumina ; but the lime, iron-oxide 

 and magnesia are more variable, which may represent the true 

 condition of the case, or may be due to imperfect methods of 

 analysis. 



It is evident from these diagrams that the lavas of the Andes 

 represent various phases of the differentiation of a magma which 

 is chemically similar to that which has furnished the lavas of the 

 Great Basin and extreme western Cordilleras in the United States, 

 and that they belong to similar petrographical provinces. 



Joseph P. Iddings. 



