324 F, BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



The presence of water, which characterizes them as sediments, is favor- 

 able to crystallization rather than to granulation and rotation. The 

 intensity of the pressure also favors crystallization rather than granu- 

 lation. 



The earth movements, which metamorphosed the sedimentary series? 

 were accompanied by the intrusion into the rising crust of the earth of 

 great igneous masses which further metamorphosed the squeezed and 

 crushed sediments and which consolidated into granite, gabbro, pyrox- 

 enite, and peridotite. If there were any volcanic outbursts at this time, 

 all record of them has been removed by erosion. 



The igneous material that is now exposed consolidated before reaching 

 the surface. Pressure was a much less active agent in the further crys- 

 tallization and metasomatism which accompanied the intrusion of these 

 igneous bodies. The crystallization was therefore of a more granular 

 character and the new minerals are not those which possess less bulk 

 than the original minerals. Under these conditions garnet, tourmaline, 

 staurolite, andalusite, hornblende, and chlorite have developed. 



These minerals reach a size which renders them conspicuous to the 

 naked eye. When developed in the neighborhood of igneous bodies they 

 take part in the parallel orientation common to the other constituents 

 of the gneiss. 



Their longer axes or their prominent cleavages may be transverse to 

 the schistosity of the containing rock. This fact, the relatively larger 

 dimensions of the crystals, and their granular form give them a porphy- 

 ritic character and show that their formation must have been subsequent 

 to the period of greatest pressure. 



With the elevation and accompanying metamorphism, the character 

 of which has been indicated, of these simple sediments began the history 

 of the Piedmont land surface. 



It was stated on page 291 that the present form of the plateau is not a 

 constructional form ; that the rock structure is discordant with the sur- 

 face configuration. If the comparatively slight depressions made by the 

 present streams were filled in, a perfectly level plain would result, sloping 

 seaward. If, on the other hand, the arches and troughs of the original 

 anticlines and synclines were restored, a region of lofty mountains and 

 deep valleys would be developed. These two strikingly unlike topog- 

 raphies are separated by a long and more or less complex erosion his- 

 tory. It is to be doubted whether the perfect constructional form ever 

 existed, but a greater height of land than exists at present must have 

 characterized this region during some portion of Paleozoic time. This 

 height of land was]so far reduced at the opening of Triassic time that it 



