338 E. B. MATHEWS — MARYLAND AND PENNSYLVANIA PIEDMONT 



manifestly the metamorphosed representatives of surface volcanoes. 

 The western limits of the slightly altered phyllitic rocks and their rela- 

 tion to the sedimentary rocks of known age at the base of Catoctin moun- 

 tain have thus been described by Professor Williams : * 



" If we follow the succession of strata eastward from Catoctin mountain, which 

 bounds the Piedmont plateau on the west, we find above the hard Cambrian sand- 

 stone a little limestone, which, however, is immediately buried beneath the over- 

 lap of red sandstone. This blue limestone, whose fossils show it to be of lowest 

 Silurian (Trenton) age, soon, however, emerges from beneath the overlying and 

 unconformable Triassic (Newark) sandstones as a series of considerably folded 

 beds, which are succeeded on the east and apparently overlain by carbonaceous 

 and hardly altered shales. These are like those which occupy a similar position 

 above the same limestone farther westward [Martinsburg shales]. Still beyond 

 there follow with the earterly dip the thick beds of sandstone which compose 

 Sugarloaf mountain. . . . The Sugarloaf sandstone passes on its eastern side 

 upward by a gradual transition to shaly layers into sandy slates, and these again 

 into the succession of sericite and chloritic schists, which compose the mass of the 

 semi-crystalline area." 



CARDIFF QUARTZITE 



The quartzite and quartzose conglomerate which occur in the north- 

 east part of Harford county form a narrow band apparently resting on 

 the phyllite and underlying the Peachbottom slate, wrapping around 

 the latter and extending beyond its southwestern limits to the valley of 

 Broad creek. The formation is of small thickness and of slight extent. 



PEA CHB TTOM SLA TES 



The Peachbottom slates extend as a narrow strip within the limits of 

 the Cardiff quartzite and pass beyond it across the Susquehanna river 

 into Lancaster county, Pennsylvania. The formation is composed en- 

 tirely of characteristic blue-black slates, similar to the material put on 

 the market, and the homogeneity of the formation is now so complete 

 that it is impossible to perceive within it any succession of sedimentary 

 beds. It is usually considered, however, that the central portion of the 

 ridge differs somewhat from the sides, and that the formation represents 

 the uppermost member in a tightly pinched syncline. 



The age of these Peachbottom slates has been somewhat questionably 

 determined on doubtful fossil evidence to belong to the Hudson River 

 horizon of the Ordovician. If this correlation is correct, we have an 

 upper limit fixed for the age of the deposits of the eastern Piedmont in 

 Maryland, and an apparent confirmation of the position assumed that 



* Maryland, Its Resources, Industries, and Institutions. Baltimore, 1893, p. 60. 



