Page:The Journal of geology (IA journalofgeology11893univers).pdf/397

The identification of acid and basic volcanic rocks in the South Mountain, Pennsylvania, has already been announced.^[1] This announcement has been further substantiated by detailed petrographical study which it will be the purpose of a later communication to discuss. The present discussion of these rocks will be limited to the acid volcanics, and its object will be; a) to show that the acid volcanics were originally identical with their recent volcanic analogues; b) to further show that their present differences are due to changes subsequent to solidification, chief among which has been devitrification; and c) to propose a name for them that shall express these facts. The structures, which will be described in the course of the paper, will be considered a sufficient guarantee of the igneous origin of the rocks which possess them, without further proof on that point.

Three distinct rock types have been recognized in the South Mountain. (1) A silicious sedimentary formation, represented by a quartzose conglomerate, a sandstone, and a compact quartzite. This is rarely accompanied by an interbedded argillaceous slate. The age of these sediments has been recently determined as lower-Cambrian by Mr. Walcott^[2] from the discovery of fossils in the interbedded slates. Underlying these Cambrian sediments, but exposed by erosion for many square miles (150-175), are two types of volcanic rocks, distinctly different in chemical composition but affected by like conditions of con-