Page:The Journal of geology (IA journalofgeology11893univ).pdf/182
because of the greater or less completeness of the material collected, than by reason of their actual scarcity or abundance in nature.
The chief feature of the report consists in the systematic description of the lavas of Colombia based upon their microscopical investigation in conjunction with their chemical analysis. The second part of the report is devoted to a description of the rocks in connection with their geographical distribution. It is to be regretted that the geological relations of the rocks with one another are not furnished at the same time.
The rocks are first discussed from a mineralogical standpoint, their mineral composition and structure being taken as the basis of classification within the general group of extrusive igneous rocks, to which they all belong. They are all embraced within the families of andesite and dacite, as defined by Rosenbusch. They present a chemical series grading from rocks relatively poor in silica and rich in lime and magnesia with sodium considerably in excess of potassium, to those comparatively rich in silica, and poor in lime and magnesia, but with sodium still in excess of potassium. The lower limits approach basalt, and the upper limits border rhyolite.
The same gradual transition exists in the mineralogical composition. At one end are pyroxene-andesites with accessory olivine, the feldspars being rather basic plagioclase. These pass into pyroxene-andesites without olivine, and into hornblende-pyroxene-andesites, and hornblende-andesites, and with increasing amounts of quartz into dacite, or quartz-andesites. In the dacites the feldspars are: plagioclase, approaching albite, and sanidine; while biotite becomes prominent among the ferromagnesian minerals.
In considering the classification of such a series of rocks, since their mode of occurrence is the same throughout, namely, that of lava streams, Küch finds the grounds of classification to be: chemical composition, mineral composition and structure. Of these, chemical composition is undoubtedly that which under like conditions of solidification controls the mineral and structural