64 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
of the coloring matter parallel to tlie boundaries of the fragments, due 
to water deposition. Grossed nicols again show uniform crystallization 
or a micropoikilitic structure. In one specimen of breccia this was not 
the case, however. A coarsely crystalline siliceous cement is quite 
distinct in grain from the uniformly finely crystalline fragments. This 
maybe a lava flow crushed and recemented. 
TUFFACEOUS BRECCIA. 
In some instances the groundmass doubtless represents an altered 
ash, when the rock becomes a true breccia. A breccia of this sort from 
the Monterey district has epidote largely developed in the matrix. 
The granulated quartzes and the perthitic feldspars of the included 
fragments show in a marked way the effect of dynamic action. 
At Raccoon Greek a fine specimen of breccia was found (PI. XIII), 
and in the Buchanan Valley breccia is extensively exposed. Some of the 
fragments contain chain spherulites. At Goles Gorner (Buchanan Val- 
ley), some G miles northeast of Graeffenburg, the breccia is sheared, 
and with the development of sericite the rock has become more or less 
slaty, Avhile still conspicuously retaining its brecciated character. 
The presence of these tuffs, flow breccias, and breccias proper, which 
are the natural accompaniments of surface lava flows, is inexplicable 
under any other hypothesis of the origin of the acid rocks. 
METAMORPHOSED ACID ERTTPTIVES : SERIOITE-SCHISTS AND SLATES. 
The alteration of the feldspathic constituent of quartz-porphyries to 
sericite or some other micaceous mineral, under the action of dynamic 
forces, has been frequently described. 1 
The production, in this way, from massive acid eruptives, of schists 
and slates resembling true porphyroids, 2 is finely illustrated in the 
South Mountain. In a single exposure (west end of Long Mountain, 
west of Gettysburg) felsiteswith distinct phenocrysts grade insensibly 
into a crinkled sericite slate. The shear zone is of limited width (10 
feet), and bounded on each side by massive felsites. The phenocrysts 
are only slowly obliterated and can be distinguished until the last stage 
in the alteration has been reached. 
Thin sections of five successive stages were studied. They show a 
development of sericite first around the feldspar phenocrysts and in a 
plane of dislocation. It is only sparingly developed in the ground- 
J J. Lehmann, Untersuchungen iiber die Entstehung der altkrystallinischen Schiefergesteine, Bonn, 
1884, Cap. IX, Druckschieferung und Glimmerbildung, p. 136. 
A. von Groddeek, Zur Kenntniss einiger Sericitgesteine, welche neben und in Erzlangerstatten 
auftreten : Neues Jakrbuch fur Mineral., etc. Supp. Vol. IV, 1886, p. 428. 
G. H. Williams, Bull. II. S. Geol. Survey No. 62, pp. 61, 121, 212. 
Bonney, On some nodular felsites in the Bala group of North Wales: Quart. Jour. Geol. Soc. Lon- 
don, Vol. XXXVIII, p. 289. 
Callaway, On the genesis of the crystalline schists of the Malvern Hills: Quart. Jour. Geol. Soc. 
London, Vol. XLI1I, pp. 530, 531. 
P. L. Milch, Beitrage zur Xenntnis des Verrucano, 1892, pp, 128, 129. 
2 Eosenbusch, Pet. Massigen Gesteine. Vol. II, 2d ed., p. 411. 
