DISTRIBUTION AND MODE OF OCCURRENCE 625 



the west in coarse quartzose conglomerate and limestone conglomerate. 

 These two divisions probably represent the Brunswick shale of eastern 

 Pennsylvania, as the Lockatong type of sediment apparently does not 

 reach this region, but ends south of Pottstown, Pennsylvania, as described 

 by A. C. Hawkins. 2 The Triassic is abruptly terminated on its western 

 edge by a normal fault. 



Assuming that the beds are not repeated by faulting, as no evidence of 

 strike faults was observed within the Triassic of this area, it is com- 

 puted from the dip of the beds that there is represented here 23,000 feet 

 of strata, but at no place in the area does this thickness occur. At the 

 western edge of the basin, where, because of the continuous westward dip 

 of the beds across the basin and the great drop fault, the greatest thick- 

 ness of strata should be expected, the Triassic deposits are very thin, and 

 the limestone floor on which they rest is exposed in the lowlands near 

 York Springs and about Fairfield for nearly 3 miles horizontally under 

 the Triassic. Although the floor of the basin is believed to descend much 

 more rapidly east of these places, as shown in the sections (figure 2), 

 nowhere in the basin will the thickness of the beds approximate the total 

 thickness of the deposit, because the beds successively overlap westward 

 mi the limestone floor. 



IGNEOUS HOCKS 



The main igneous mass in the area is -the great Gettysburg sill, which 

 begins at Emmitsburg, Maryland, a short distance south of the Pennsyl- 

 vania line, passes northeastward near Gettysburg, and ends a short dis- 

 tance east of Dillsburg. It has an average width of over 1 mile and ex- 

 pands to over -2 miles in places. By its relations to the sedimentary rocks 

 it is shown to be a sheet intrusive in the westward dipping strata, ils 

 upper surface being clearly exposed in the Gettysburg battlefield reserva- 

 tion dipping 20° northwestward under the shale. Its general thickness 

 is about 2,000 to 2,500 feet. That the sheet is somewhat cross-cutting, 

 however, is indicated by the fact that it gradually rises in its position in 

 the beds southwestward, as shown by the stratification lines on the map 

 and by minor irregularities of its outline and width, which are believed 

 to be due to cross-cutting. Some of the more regular offsets, as those 

 east of Gettysburg, are probably caused by faulting. The marked expan- 

 sion in width in the northeastern part of the Gettysburg quadrangle is 

 apparently a local thickening of the sill, accompanied by cross-shearing 

 of the sedimentary beds and greater uplift of the covering strata. 



2 A. C. Hawkins : Dockatong formation of the Triassic of New Jersey and Pennsylva- 

 nia. Annals N. Y. Acad Sci., vol. xxiii, 1914, pp. 145-176. 



