THE VOLCANIC ROCKS OF THE ANDES. 1 65 



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 Stubel from Ecuador, led Kiich 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 Kiich 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 Kiich's interpretation. Dacite, though 

 seldom mentioned by previous investigators, is of very frequent 

 occurrence judging from the collections by Reiss and Stubel. 



The study of such abundant material naturally led Kiich to 

 first treat the lavas of Colombia as one general group of inti - 

 mately 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 



