1920.] NATURAL SCIENCES OF PHILADELPHIA. 357 



quite fine-grained at the contacts. Three miles northwest of Leb- 

 anon, the quartz diabase is porphyritic, with black augite pheno- 

 erysts up to 1 cm. in length, in labradorite. 



The textm-e is diabasic or ophitic. The labradorite is quite 

 zoisitized, and the associated augite is more or less altered to chlor- 

 ite, all stages of the alteration from incipient changes along cleavage 

 cracks to complete chloritized individuals being shown in thin 

 section. Graphic intergrowths of quartz and orthoclase form inter- 

 stitial aggregates. Magnetite and p3a-ite are the principal accessory 

 minerals, the latter being recognizable in most hand specimens. 



Basalt. The basalts are exposed along the Swatara Creek, on the 

 north slope of Bunker Hill, along the railroad cut one half mile north 

 of Bunker Hill Station, and in the road cut one half mile east of 

 Bunker Hill Station. 



The rock is chiefly a brecciated or tuffaceous amygdaloidal bas- 

 altic glass, indicating that the flow occurred under water on the 

 floor of the Ordovician sea. The brecciated character is well shown 

 on weathering, which also causes the rock to assume a vesicular 

 appearance due to the weathering out of the calcite amygdules. 



Freshly broken specimens show angular fragments of dense black 

 glass in an aggregate of greenish glass and calcite amygdules. On 

 weathering the rock becomes dark yellow. The most typical basalt 

 occurs two miles southeast of Jonestown, where it forms a dense 

 crystalline rock, with amygules of calcite, or more rarely, of quartz. 



Under the microscope, the basalt breccia (Figs. 3-5) is seen to 

 be composed of greenish glass (n < 1.60) showing perlitic structure, 

 which exhibits strain effects or incipient crystallization under crossed 

 nicols. The glass is filled with inclusions, and larger fragments of 

 crystalline basalt, consisting of aggregates of plagioclase laths and 

 augite in a dark glassy gi'oundmass, similar to the crystalline basalt, 

 two miles southeast of Jonestown. 



