PETROGRAPHY 637 



Basaltic diabase, or basalt. — Basaltic diabase, fine grained to dense and 

 nearly black, constitutes the dikes and thin sheets and the contact facies 

 of the larger masses. Scattered phenocrysts of pyroxene and less com- 

 monly of feldspar and olivine are found in many places; otherwise the 

 .texture is entirely aphanitic. This facies of the rock, particularly where 

 a glassy base persists, is essentially a basalt similar to that which consti- 

 tutes the lava flows of the same magma in the Watchung Mountains of 

 New Jersey. In the holocrystalline type the texture is prevailingly dia- 

 basic, with pyroxene generally in excess of feldspars. Abundant magne- 

 tite, uniformly disseminated through the rock in the form of minute 

 granules, accounts in large measure for the black color. Locally, larger 

 grains and crystals of magnetite occur, some in beautiful skeleton form. 

 Minor accessories include a little biotite, minute needles of apatite, and 

 scattered phenocrysts of olivine in places or serpentine pseudomorphs of 

 this mineral. 



Olivine basalt. — Basaltic diabase grades into t} 7 pical basalt, as stated 

 above, and this occurs both with and without scattered grains of olivine. 

 In places this mineral is so abundant as to constitute typical olivine ba- 

 salt. The thin sheet east of Chestnut Hill is also vesicular at the top. A 

 clark-brown glassy base, thickly sprinkled with granules and skeleton 

 crystals of magnetite, is abundant in a specimen from a thin sheet south- 

 east of Bendersville. In places the magnetite is so abundant as to render 

 the glass black and opaque. 



PETROGRAPH1G DETAILS 



Pyroxene. — The common monoclinic pyroxene is nearly black in the 

 hand specimen, but is pale green to almost colorless in thin sections. Ex- 

 ceptionally it exhibits a slight pleochroism in shades of pale green and 

 light greenish yellow. Having generally crystallized later than the feld- 

 spars, it has no crystal boundaries. The elongated prisms in some coarse 

 pegmatitic varieties and the scattered phenocrysts in some of the basaltic 

 facies are exceptions. Common types of twinning are those parallel to 

 (1) the orthopinacoid (100), producing paired halves; and (2) the basal 

 pinacoid (001), in repeated thin lamellae, probably due to stresses in the 

 rock. Magnetite and biotite, where it occurs, are common inclusions, and 

 here and there minute apatite crystals, although the latter is more abun- 

 dant in the feldspars and quartz. 



Some degree of alteration to uralitic amphibole or serpentine, less com- 

 monly to chlorite, is almost universal. One of these secondary products 

 usually predominates almost to the total exclusion of the others, although 

 all of them are found together in some sections. The uralite in turn 



