630 STOSTC AND LEWIS TRIASSIC IGNEOUS ROCKS 



duction of pressure within the deeper-seated rocks sufficient to permit 

 some of them, whose temperature was above their melting points, to 

 liquefy. The molten material, still under pressure, but now free to move, 

 followed the line of least resistance — upward in the zone of reduced 

 pressure — and made its way along the joints, crevices, and fault-planes 

 in the floor of the Triassic basin into the Triassic sediments. Here it 

 wedged in between the northwestward-dipping beds and formed a great 

 sill, while part of it ascended along the northwest base of the sediments 

 between them and the flat floor of Paleozoic limestone, forming another 

 great sill. The overlying strata between these sills was at the same time 

 broken through by great cross-cutting bodies of the igneous mass and by 

 smaller bodies that followed joint planes in the form of dikes. The 

 elongated vent up which the molten magma came was probably at the 

 western margin of the more deeply sunken portion of the basin, where 

 the latest fracturing and faulting of the floor of the basin would have 

 occurred. The further sinking of the Triassic basin resulted in a great 

 irregular fracture along its northwestern side, along which the whole 

 Triassic block settled several thousand feet, carrying the Paleozoic lime- 

 stone floor far below the base of the Cambrian quartzites of South Moun- 

 tain. 



It seems probable that the molten rock of the large sheets and cross- 

 cutting bodies must have reached the surface, unless the thickness of 

 Triassic sediments was much greater than is supposed, and poured out as 

 lava, as it did in the northeastern part of the Triassic basin in New Jer- 

 sey; but such surface flows, if they did exist in this area, have been re- 

 moved by the later erosion that carried away the higher sedimentary beds. 

 A possible exception is found in the western edge of a thin sheet that 

 readies the boundary fault one-half mile south of Bendersville — the mid- 

 dle one of the three small concentric sheets. Here the igneous rock is 

 basaltic, in part highly vesicular, and much decomposed ; and there is a 

 gradation from this to red shale in which are imbedded only scattered 

 fragments from the disintegrating ledge. 



Petrography; by J. Volney Lewis 



SUMMARY 



The igneous rocks are dominantly diabasic, but show a remarkable 

 diversity of differentiation facies and a corresponding variety of physical 

 characteristics. They range from coarse-grained granitic texture and 

 pink to light or dark gray colors in the larger bodies to dense black rock 

 in the contact facies and in thin sheets and dikes. 



