316 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



differentiation has given rise to representatives of many classes, the spe- 

 cies show distinct magmatic relationship. 



The intrusives of the Pennsylvania Piedmont are obviously part of a 

 zone of docalcic or percalcic igneous types, a zone which, as pointed out 

 by Doctor Washington, extends parallel to the Atlantic coast and lies 

 west of a zone of dosodic to persodic igneous rocks. 



SERPENTINES {META-PYROXENITE, META-PERIDOTITE, AND OTHER 

 ALTERA TION PROD UCTS) 



Distribution. — More or less intimately associated with the Atlantic belt 

 of gabbro are altered pyroxenites and peridotites or meta-pyroxenites 

 and meta-peridotites. These are the serpentines and related rocks. Ser- 

 pentines of such origin are found in the Piedmont belt of North Carolina, 

 Virginia, Maryland, Delaware, Pennsylvania, and New York states. 



In the Philadelphia district there are four belts of interrupted lenticu- 

 lar exposures of serpentine and closely related rock types trending north- 

 east and southwest. Two of these belts are on the southeast flank of the 

 gabbro ; one belt lies within the gabbro area and the fourth belt lies on 

 the northwest flank of the gabbro. All of these belts continue south- 

 westward into Delaware and Maryland. 



Serpentine country, even though of so limited an area as is the case 

 in the Philadelphia district, possesses certain marked and characteristic 

 features. The areas underlain by serpentine stand out in relief as low 

 ridges. The rock, though very soft, is exceedingly stable chemically, and 

 hence weathering leaves it in relief. For the same reason the rock lies 

 close to the surface of the ground, with many outcrops. In the less 

 altered peridotites the material weathers in huge boulders, which strew 

 the ground. Where the peridotite has been completely altered to serpen- 

 tine the surface of the ground is covered with flat fragments broken along 

 joint planes. The soil is thin and, like the rock, of a light green color. 

 On relatively low land, where the soil possesses greater depth, it is of a 

 deep-red color, due to the oxidation of the iron silicate. 



Serpentine soil, because of its thinness and because of the magnesia 

 which it contains, is not a fertile soil, and vegetation on serpentine areas 

 is therefore scanty and of a peculiar character. The moss pink (phlox 

 subulata) covers the ground in such profusion as to locally give the 

 name Pink hill to some of the serpentine ridges. Cedars, scrub-oak, and 

 cat-brier are very characteristic of the serpentine country, which usually 

 presents a wild and barren aspect. 



Character. — Of the four dikes the most southeasterly, which extends 

 from Bryn Mawr to Chestnut hill, lies wholly in the Wissahickon mica- 

 gneiss, but is presumably an offset from the main mass. Both the field 

 and petrographic characters of this formation are those of an igneous 



