624 STOSE AND LEWIS TRIASSTC IGNEOUS ROCKS 



Page 



Titanite 639 



Tachylite, basalt glass 639 



Chemical composition 639 



Composition of various types of diabase 639 



Composition of the pyroxene 641 



Composition of the feldspars 641 



Mineral composition 643 



Distribution and Mode of Occurrence; by George W. Stose 



INTRODUCTION 



The area here described and shown on the map (figure 1) covers the 

 Fairfield, Gettysburg, and Carlisle quadrangles and the immediately ad- 

 jacent region. These three quadrangles were surveyed by the writer for 

 publication in folios of the geologic atlas of the United States. The map- 

 ping of the adjacent region is taken from published State geologic maps 

 of Maryland and Pennsylvania somewhat modified. 



The Triassic igneous rocks of this area, with the possible exception 

 of a thin sheet just south of Bendersville, are all intrusive diabase and 

 include one large sheet, several cross-cutting bodies, and many small 

 sheets and dikes. Their distribution is shown on the accompanying map, 

 figure 1. The concealed and weathered condition of the exposures make 

 it difficult to determine the mode of occurrence of some of the masses, 

 but the relations shown in the sections (figure 2) are probably correct 

 for the portions near the surface. The vent shown on the sections through 

 which the molten magma is believed to have ascended is of course hypo- 

 thetical, but is based on the facts described and the conclusions reached 

 in this paper. 



SEDIMENTARY ROCKS 



The Triassic sedimentary rocks of this area almost invariably strike 

 northeast and dip northwest from 10° to 35°, with an average of 15° to 

 20°. Along the western border, although local pronounced clips were ob- 

 served, the bedding is on the whole nearly horizontal. The lower beds at 

 the east are reel micaceous sandstones and sandy shales interbedded with 

 harder light-gray micaceous sandstones and arkose. At and near the base 

 there are a few thin beds of quartz conglomerate, limestone conglomerate, 

 and black fissile shale. This division is the representative of the Stockton 

 formation of eastern Pennsylvania and New Jersey. The middle portion 

 is chiefly soft red shale. The upper portion comprises both hard and 

 soft red sandstones and shale and harder gray sandstone, culminating at 



