THE ANCIENT VOLCANIC ROCKS OF SOUTH 
MOUNTAIN, PENNSYLVANIA. 
By Florence Bascom. 
INTRODUCTION. 
The mountain range known in Vermont as the Green Mountains, in 
Massachusetts as the Taconic Mountains, and in New York and New 
Jersey as the Highlands, is the South Mountain in Pennsylvania and 
Maryland, the Blue Eidge in Virginia, and the Smoky Mountains in 
North Carolina. The South Mountain in Pennsylvania lies just east 
of the middle of the State, and stretches from Maryland north and 
east in a sickle-shaped curve toward the Susquehanna, While in the 
! New England States the mountains of this range rise to heights from 
3,000 to 4,000 feet above sea level, and in its southern extension from 
4,000 to 7,000 feet, in Pennsylvania its summits rarely exceed 1,500 feet. 
South Mountain is 50 miles in length, and 10 miles wide at its broad- 
i est point. It covers from 150 to 175 square miles, and abundantly 
exposes distinct and interesting rock types. These rocks, prevailing 
throughout the entire South Mountain range, have long been known 
to geologists, although their true character was not recognized until 
recently. 
In December, 1892, as the result of field work on the part of Dr. G. 
I H. Williams in the northern and of the writer in the southern portion 
I of South Mountain, there appeared a preliminary description 1 of two of 
the rock types, in which their identification as ancient volcanics was 
announced. In this bulletin it is proposed to substantiate that identifi- 
cation with more detailed proof. It is further proposed to show that 
these ancient igneous rocks were, at the time of their consolidation, 
identical in character with their recent volcanic analogues, and that 
their present differences are due to subsequent changes, chief among 
which has been devitrification. It is also proposed to recognize these 
facts in the nomenclature. 
The petrographical features of the third and only remaining rock 
type of the South Mountain will also be described in some detail. 
A brief report upon the previous geological work accomplished in 
the South Mountain, and some account of the structural features of 
the mountain and the age of its rocks, will precede the petrographical 
discussion. 
'The volcanic rocks of the South Mountain in Pennsylvania and Maryland: Am. Jour. Sci., 3d 
series, Vol. XLIV, Dec, 1892, pp. 482-496, PI. I. lleprinted in the Scientific American for Jan. 14,1893. 
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