Petrografic description of some igneous rocks from the Cordillera of South Mendoza, in the Argentine

Comparatively little is yet known of the intricate igneous history of the Cordillera of South Mendoza. Almost all the knowledge we have is due to the field- and stratigraphical work of Dr. H. Gerth of Leiden (bibl. 1 and 2), and the petrographic studies of Dr. H. G. Backlund (bibl. 3 and 4) on the material collected by the former investigator. A part of this material, however, had remained unexamined at Leiden, and at the request of Dr. Gerth Dr. L. U. de Sitter began an examination of the 25 slides prepared. After a provisional examination and the determination with the universal stage 1) of some plagioclases in most of the slides that were not too much decomposed Dr. de Sitter left Leiden for Dutch East India. Dr. Gerth was so kind as to allow me the further examination, and I have great pleasure in expressing my sincere thanks to him for entrusting me with the valuable material. All data concerning the geological occurrences and field-evidence were supplied by Dr. Gerth. I myself, not having the use of a theodolite microscope for the time being, Mr. Ch. Harloff was so good as to make some further additions to Dr. de Sitter's determinations (slides n°. 970, 9, and 28, with the accompanying drawings). The petrographic descriptions are on my own responsability. In a few cases I thought it better to give other names to rocks, that were identical with, or very closely related to specimens already described by Backlund, taking Holmes’ “Nomenclature of petrology” as a basis. For a pre-tertiary andesite the somewhat antiquated term of “porphyrite” has not been employed. For the micrographs I used rough tracings made with an optical bench for micro-projection. The general shape and relative sizes of the larger items are therefore accurate.