320 C. R. KEYES— SECTION ACROSS THE PIEDMONT PLATEAU. 



the widely spread Leptcena sericea, SoAverby. Meek's 

 Orthis des7nopleura from the Chazy of Colorado seems 

 to be very closely related. The species here noticed are 

 characteristic of the Trenton period. This fact, taken 

 in connection with the stratigraphic position of the 

 fossiliferous horizon, leaves little room to doubt that the 

 strata here referred to are of Trenton age. But, as al- 

 ready intimated elsewhere, the entire series of limestones 

 and shales between the two great sandstones of the 

 Catoctin and Sugarloaf mountains probably represents 

 the Chazy, Trenton and Hudson River formations of 

 the more northern localities. 



In passing westward along the line of the section, the 

 Piedmont plateau gradually increases in elevation above 

 sea-level from about 200 feet on the eastern border to 

 nearly 600 feet at the base of Sugarloaf mountain. This 

 prominence rises abruptly to a height of almost 1,300 

 feet above mean tide. The broad Frederick valley 

 beyond has an elevation scarcely higher than the eastern 

 part of the plateau. Lastly comes the Catoctin range, 

 which rises nearly to an equal height with Sugarloaf. 

 Both mountain crests are formed of hard sandstone, 

 shown in thin sections to be of unmistakableclastic origin 

 (figure 4). The rocks between are contorted limestones 

 and slates, the former being overlain by Newark (Triassic) 

 sandstone for more than half its supposed extent. The 

 Sugarloaf sandstone is seemingly identical with that of 

 the Catoctin. By a double thrust it apparently presents 

 twice its actual thickness, the upper member forming 

 Sugarloaf itself, and the other member forming a some- 

 what lower elevation immediately west of the mountain. 

 By the intercalation of numerous thin argillaceous bands 

 the great sandstone rapidly loses its sandy character and 

 passes gradually into typical phyllites. These schistose 

 rocks, in broad alternating hydromicaceous and chloritic 

 belts, have a superficial extension half way across the 

 plateau. At first the cleavage planes have a low angle 

 and are parallel with the inclination of the great sand- 

 stone ; but gradually the inclination becomes greater 

 and greater until near the axis, at Derwood station, it is 

 perpendicular. Near Sugarloaf these planes have the 

 appearance of being coincident with the lines of strati- 



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