Page:The Journal of geology (IA journalofgeology11893univ).pdf/180

Through the excellent work of Dr. Richard Küch,^[1] who has recently published the results of his investigation of the rocks collected by Reiss and Stübel in Colombia, we are put in possession of some important conclusions regarding the character of all the volcanic lavas of the South American Andes. Most of these conclusions are pointed out by Dr. Küch in the work cited; to these the present writer wishes to add a few not heretofore noted.

In order to appreciate the value of Küch's work, it should be observed that it was carried on upon the very extensive material collected by Reiss and Stübel during a prolonged exploration of the high mountainous regions of South America, in which they visited Colombia, Ecuador, Bolivia, Peru and Chili and brought away with them 18,000 specimens. In some places as many as 800 were collected, in others much fewer; for, as Reiss observes in the introduction to the volume upon Colombia, many of the mountains are well nigh inaccessible, their bases being covered with dense forest, and their summits hidden beneath snow and glaciers, and shrouded with clouds the greater part of the year. This is equally true of the Cordilleras farther south, so that the exploration of the region is attended with great difficulties. And while it is not claimed that the collections are complete, they must certainly be taken as representatives of the whole of the Andes.

The volcanoes of Colombia chiefly occur along the crest of the central range, rising above crystalline schists, and eruptive masses in the Cretaceous formation, whose upturned strata compose the ranges east and west of the central Cordillera. Heretofore, with few exceptions, the volcanic rocks examined have