320 F. BASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



near Chelten avenue, Germantown ; an exposure on the Pennypack 

 creek, north of Holmesburg, reveal the character of the material. It is 

 also penetrated by the shafts and tunnel of the Torresdale filtration 

 plant. The contact of the hornblende-gneiss and the Wissahickon 

 mica-gneiss was seen in this tunnel. The contact is diagonal to the 

 structure planes of the mica-gneiss, and apophyses from the hornblende- 

 gneiss extend into the mica-gneiss. The character of this contact and 

 the constitution of the rock indicate an intrusive, igneous origin. The 

 rock is medium-grained and dark colored, owing to the prevalence of 

 hornblende. This constituent is dark green and is arranged with the 

 longest axes parallel, thus producing the gneissoid structure. The other 

 constituents are quartz, orthoclase, microcline, oligoclase, biotite, titanite, 

 and apatite. The structure is granulitic and gneissoid. Owing to the 

 excess of hornblende, the rock is darker colored than the granitic intru- 

 sive on the west side of the Schuylkill river. It is otherwise not unlike 

 it, and may be genetically related to the granite. The stone, which is 

 extensively quarried at Frankford, has been used for building purposes 

 and for bridge abutments. 



DIABASE 



The intrusion of igneous material, which characterized the Triassic 

 period and which manifests itself to the north in the great diabase 

 masses of mount Holyoke, mount Tom, the Palisades, and First and Sec- 

 ond mountains in New Jersey, shows itself in this district in a few dikes 

 not exceeding 100 feet in width. The chief of these dikes has been known 

 as the Conshohocken dike because of a prominent outcrop of it in that 

 town. This exposure is figured in plate 63. It has been traced contin- 

 uously from Lafayette hill to the gulf, and thence intermittently into 

 Maryland. It possesses a width of about 30 feet. 



The rock exhibits a very uniform character. The boulders are readily 

 recognized by a rusty-yellow oxidized coat and a greenish-gray color on 

 the fresh surface of the characteristically conchoidal fracture. The rock 

 is a comparatively fresh, medium-grained, typical diabase, with plagio- 

 clase and pyroxene in about equal amounts as primary essential con- 

 stituents and ilmenite and apatite as accessory constituents. The sec- 

 ondary constituents are chlorite, scanty biotite, quartz, calcite, and 

 epidote. The plagioclase is labradorite-bytownite, which forms a net- 

 work of automorphic lath-shaped crystals. The pyroxene is the alumi- 

 nous monoclinic variety, augite. It is zenomorphic and fills the inter- 

 stices of the feldspar network. Another diabase dike 80 to 100 feet wide 

 extends northeast and southwest in the neighborhood of Dreshertown, 

 Jarrettown, and Warminster. It is exposed in a cut on the Pennsylva- 



