in Pennsylvania and Maryland. 493 



and the result bears a superficial resemblance to the well- 

 known triassic breccia ("Potomac marble," "calico rock") of 

 the Frederick valley. The material in this case, instead of 

 being limestone, is entirely rhyolite, and exhibits remarkable 

 variety of structures and colors. Both flow- and tuff-breccias 

 occur here, while a portion of the mass has been sheared into 

 a quite fissile slate. Similar acid tuffs, though of less striking 

 appearance, occur at many other points (Raccoon Creek, Mon- 

 terey, old Furnace road, etc.), and will doubtless continue to 

 be discovered as the examination of the region proceeds. 



Fragmental deposits consisting wholly of basaltic material 

 abound along the Western Maryland railroad near Monterey, 

 and farther south. The finer cementing material is in these 

 almost always altered to epidote. It is also not uncommon to 

 find coarse breccias consisting of both the acid and basic types 

 of rock, but a careful search has thus far failed to discover 

 any fragments of sandstone in these pyroclastic beds 



4. Geological occurrence and relations to the Sandstone. — 

 No evidence is necessary, beyond the petrographical charac- 

 ters above described, to establish the igneous and volcanic 

 nature of the South Mountain rocks. Additional evidence of 

 a purely geological kind is not, however, wanting. The vicis- 

 situdes through which these ancient rocks have passed, and 

 their present inadequate exposure, tend to obscure their orig- 

 inal relationships. Nevertheless dykes may be seen at various 

 points, especially at the western end of the railroad tunnel on 

 Jack's Mountain, where an amygdaloidal red felsite cuts the 

 massive and schistose greenstones. Further exploration will 

 doubtless bring to light many similar occurrences. Successive 

 flows are not now easy to separate, but the amygdaloidal and 

 fluidal structure of the rocks indicates that they must have 

 been extruded in this form. 



The age of the South Mountain volcanics and their relations 

 to the sandstone in which Mr. Walcott has recently identified 

 the lower Cambrian fauna, are points of great interest. The 

 hypothesis of the Pennsylvania geologists that the green- 

 stones and felsites lie above the sandstone is evidently incor- 

 rect. It may, however, be regarded as an open question 

 whether the volcanic rocks represent a much older horizon, 

 which was already eroded before the sandstone was deposited, 

 or whether they were, in part at least, contemporaneous with 

 the sandstones. 



The entire absence of sandstone as inclusions in the lavas, 

 as well as in all the accumulations of pyroclastic material ; 

 the observations of Keith, Geiger* and Walcott,f that the 



* Bull. G-eol. Soc. America, vol. ii, p. 155 ; plates 4 and 5. 

 f This number of this Journal. 



