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



South Mountain rises about fifteen miles west of Harris- 

 burg and extends, as may be seen on the accompanying map, 

 in a great sickle-shaped curve to the Maryland line. Here it 

 divides into two parts, known as Catoctin Mountain and the 

 Blue Ridge, which diverge at a small angle and enclose the tri- 

 angular Middletown valley, north of the Potomac. 



During the past summer the writer devoted considerable 

 time to mapping the volcanic rocks of this region and to col- 

 lecting suitable material for laboratory study. This has at 

 present only been fairly entered upon, so that subsequent com- 

 munications giving more detailed results, may be expected. 



2. Supposed sedimentary origin of the South Mountain 

 Volcanic rocks. — As far as is known to the writer, volcanic 

 rocks have not hitherto been definitely described as such in the 

 Appalachians. The rocks here under consideration have long 

 been known to geologists, but they have before, with the ex- 

 ception of a few of the most massive greenstones, been gen- 

 erally regarded as of sedimentary origin. 



Professor Henry Rogers in 1858 speaks of South Mountain 

 southwest of the Susquehanna, as embracing " a singularly 

 small amount" of igneous rock. He alludes to the cleaved 

 greenstones as " dark green slate," and to the acid porphyries 

 and felsites as " highly metamorphic Primal slate." He con- 

 tinually contrasts the highly altered slate and the unaltered 

 sandstone,* although in reality one rock is scarcely more 

 changed than the other. 



Philip Tyson, in his first report as State Agricultural 

 Chemist made in 1860, speaks of the sandstone of South 

 Mountain as Potsdam and says that it contains fossilized stems 

 of plants. He also says : " A slate, varying in color from gray 

 to brownish and greenish, is ranked as an argillite, but portions 

 of it assume a marked talcose appearance, especially in Catoc- 

 tin mountain, where it has been much disturbed and altered 

 by proximity to intrusive rocks. These last consist of amphi 

 bolites (trap), porphyries, amygdaloid, serpentine and epi- 

 dote."f 



In 1877 appeared the results of Dr. Persifor Frazer's studies 

 of South Mountain. He regarded it as composed essentially 

 of a westerly (older) portion, consisting of various modifica- 

 tions of a quartz-conglomerate (Mountain Creek rock) and an 

 easterly (younger) portion composed of orthofelsite, inter- 



* Geology of Pennsylvania, vol. i, pp 203-5, 1858. 



\ First Keport of P. T. Tyson, State Agricultural Chemist, to the House of 

 Delegates of Maryland, January, 1860, pp. 34, 35 (cf. also p. 18). In his second 

 report (1862. p. 70) he gives further particulars regarding Catoctin Mountain. He 

 says "a vast mass of epidotic trap, five miles wide, was forced up from below in a 

 state of fusion. This embraces a mass of chert (rhyolite) 3000-4000 feet thick." 

 He describes the trap as carrying metallic copper, and regards it as the cause of 

 the elevation of the mountain sandstone. 



