THE STRUCTURES, ORIGIN, AND NOMENCLATURE 



OF THE ACID VOLCANIC ROCKS OF SOUTH 



MOUNTAIN. 



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 com- 

 munication 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, (i) A silicious sedimentary formation, represented 

 by a quartzose conglomerate, a sandstone, and a compact quartz- 

 ite. This is rarely accompanied by an interbedded argillaceous 

 slate. The age of these sediments has been recently deter- 

 mined 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- 



1 G. H. Williams : The Volcanic Rocks of South Mountain, in Pennsylvania and 

 Maryland. Am. Jour. Sc, XLIV., Dec, 1892, pp. 482-496, pi. I. The Scientific 

 American, Jan. 14, 1893. 



2 C. D. Walcott: Notes on the Cambrian Rocks of Pennsylvania and Maryland 

 from the Susquehanna to the Potomac. Am. Jour. Sc, Vol. XLIV., Dec. 1892, p. 481. 



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