492. G. H. Williams — Volcanic Rocks of South Mountain 



detail. Under the microscope the ophitic network of feldspar 

 laths is still fresh, and the delicate twinning as distinct as in a 

 recent lava. The form of the basaltic magnetite, of the olivine 

 phenocrysts, and of the interstitial pyroxene is also plainly 

 visible, while the arrangement of the feldspar microliths 

 among the oval vesicles clearly indicates the motion of the 

 still viscous mass. 



Fig. 8 gives two types of the basaltic structure. The one 

 on the left shows skeleton phenocrysts of olivine and a fine 

 grained, ophitic groundmass. 



Fig. 8, mag. 5 diameters. 



Left half. — Basalt with skeleton olivines, ophitic structure, fresh feldspar. 

 Arnygdules filled with quartz and chlorite. Railroad, east of Monterey. Pa. 



Right half. — Somewhat coarser grained basalt. Arnygdules filled with zeolites 

 and quartz grains. Railroad, east of Monterey, Pa. 



Both have large arnygdules which are either oval or of 

 irregular shape, and filled with different minerals. 



c) Pyroclastic deposits, tuffs and breccias. — As is generally 

 the case with large accumulations of surface eruptions, the 

 South Mountain lavas are accompanied by extensive deposits 

 of pyroclastic material. This includes coarse flow- and tuff- 

 breccias, pumiceous bombs, and banded accumulations of fine 

 volcanic ash. Like the massive rocks, this fragmental matter 

 is both acid and basic in composition. 



The most striking and important area of acid tuffs covers 

 about a square mile in the Buchanan valley, at the eastern base 

 of Piney Mountain, two miles north of the Chambersburg 

 turnpike. Here the rock is a breccia whose component frag- 

 ments vary from two or three feet in diameter to the finest 

 ash. All sizes, shapes, and colors are heterogeneously mingled, 



