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THE VOLCANIC ROCKS OF THE ANDES.
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been from the Andes south of the equator. In the present instance our knowledge of them is extended to the most northern end of the great Cordilleran system.

A critical review of all previous work upon the lavas of the Andes, and its comparison with that by himself on the lavas of Colombia, and with a preliminary study of the collections by Reiss and Stübel from Ecuador, led Küch to the conclusion that essentially the same petrographical relations exist at all the volcanoes of the Andes.

With few exceptions all of these recent volcanic lavas of which we have any knowledge, are andesites and dacites, that is, rocks whose essential constituents are soda-lime-feldspar and one or more of the minerals: pyroxene, hornblende and biotite, with which is associated quartz, in the case of dacite. Recent eruptive rocks whose mineral composition falls outside of this group appear to be of rare occurrence, and are rocks closely related to andesite and dacite in composition, namely: quartz-trachyte or rhyolite on the one hand, and basalt on the other. In two instances rocks described as trachyte and quartz-trachyte by Stelzner are shown by Küch to be more properly dacite.

The known occurrences of true basalt are few, the most basic rocks being more closely related to pyroxene-andesite than to basalt, according to Küch's interpretation. Dacite, though seldom mentioned by previous investigators, is of very frequent occurrence judging from the collections by Reiss and Stübel.

The study of such abundant material naturally led Küch to first treat the lavas of Colombia as one general group of intimately connected varieties, without reference to their geographical distribution, for it became evident, as he remarks, that neither a single rock, nor a specially abundant development of any one kind, nor the association of a certain number of different rocks could be considered characteristic of any particular volcano. The same rocks with like multiplicity of development, and the same associations of rocks, repeat themselves in different localities in such a manner that what may seem to be the prevailing or the subordinate varieties in one place are more likely to appear such

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