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Dr. Küch'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 importance, 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 relationship 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 Cordilleran 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 differentiation 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 supplying the lavas in Central America and Mexico, which are in turn less advanced than those of the United States, where volcanic activity is extinct or at least quiescent.