212 GEORGE W. STOSE 



developed in the soft shales and feldspathic rocks, but even the hard- 

 est quartzites have parallel jointing and to some extent are schistose. 

 The strike of the schistosity is parallel to the ridges, and the average 

 dip is 35° southeast. In most cases this is the dominant structure, 

 often producing a banding w^hich closely resembles bedding and 

 entirely obhterates the true stratification. It is not surprising, there- 

 fore, that many of the early geologists, mistaking the schistosity for 

 bedding, failed to understand the relations. The stratification can 

 be determined in the Scolithus-he2.rmg rocks because it is perpen- 

 dicular to the worm-tubes, ' and in the coarser sandstones and con- 

 glomerates it is sometimes shov^n by alternations of coarse and 

 fine material or by color-banding. The dips are thus observed 

 to be usually either vertical or steep to the west. The sandstone 

 series is found to dip away from, and therefore to overlie, the 

 volcanics, and to pass steeply beneath the limestone of the valley 

 along the west front of the mountain. A similar highly inclined 

 sandstone series on the east side of the volcanic rock belt is men- 

 tioned by Walcott in his paper on the Cambrian rocks of Pennsyl- 

 vania.^ The structure of the mountain belt as a whole is therefore 

 a broad anticlinorium. 



East of Montalto. — The general structure of the mountains east 

 of Montalto is a fiat-topped, steep-sided antichne, with the east 

 limb not exposed in the area. The ancient volcanic rocks exposed 

 in the low plateau at the eastern edge of the area are overlain by 

 the fiat-lying, purple, feldspathic sandstones of Snowy Mountain. 

 These sandstones may also be seen in the mountain north of the 

 volcanic area, beyond Conococheague Creek. Westward the strata 

 bend down sharply, and their upturned edges form Rocky Mountain, 

 one of the few really rugged heights in this area. The shale valley 

 in which the West Branch of Little Antietam Creek has its source 

 is followed by the Scolithus sandstone of Montalto Mountain, which 

 dips steeply to the west. At the foot of the mountain soft, calcareous, 

 Scolithus-htd^rmg (Antietam) sandstone is followed by gray and 

 mottled limestones, with vertical and steeply overturned dips. 

 Farther out in the valley the limestones are found to be closely 

 folded. 



I Loc. cit., p. 24. 



