THE VOLCANIC ROCKS OF THE ANDES, 169 



Dr. Kiich's report is to be commended, not only for its 

 scientific excellencies, but also for the form in which it has been 

 published, and the admirable indices which render its details 

 easily accessible. 



The facts brought out by this report lead the present writer 

 to certain conclusions regarding the nature .of the volcanic 

 ejections of the South American Andes of the greatest impor- 

 tance, which, however, may be modified as our information 

 becomes more complete. In brief, it appears that the main mass 

 of the lavas of all the volcanoes of the Andes is andesite, of 

 variable composition in all localities. It grades into basic 

 varieties, approaching basalt, in some places, and into acid 

 varieties which are dacites, in others. It is probable that the 

 basic varieties would be classed as basalts by many petrologists, 

 but they would not constitute the more basic forms of basalt. 

 The variability in composition and petrographical characters 

 within these limits is pronounced, and proves the intimate rela- 

 tionship between all of the lavas. The almost universal absence 

 of the most basic and most acid members of the series which 

 occur in other regions, namely, the true basalts and rhyolites, is 

 most significant, and, if established by future investigation, 

 would indicate that volcanic activity in the Andes, which is still 

 in force, had developed, by the differentiation of some magma 

 common to the whole great Cordiileran system, a series of lavas 

 of limited range. This series, though precisely similar to parts 

 of others developed in other regions, especially those of Tertiary 

 age in western North America, is wanting in the extreme forms 

 of differentiation common to the latter — that is, in basic basalt 

 and rhyolite. From this we may infer that the general differen- 

 tiation of the magma supplying the lavas of the Andes has not 

 reached its final stages, in which great volumes of extremely 

 differentiated material will have been developed. It would seem 

 to be in a much less advanced condition than the magmas sup- 

 plying the lavas in Central America and Mexico, which are in 

 turn less advanced than those of the United States, where vol- 

 canic activity is extinct or at least quiescent. 



