64 NEW YORK STATE MUSEUM 



Valley, at Rochdale and north of the Spackenkill road. The 

 Trenton also is probably present in places not yet discovered along 

 the eastern margin of this strip. At Rochdale the dark blue Trenton 

 beds have a thickness apparently between 60 and 100 feet and form 

 a conspicuous stratum. 



Strikes and dips within this strip show much uniformity. In the 

 Poughkeepsie and Eastern Railroad cut at Pleasant Valley the Tren- 

 ton beds show a strike about n. 37° e. and a southeast dip. In the 

 quarry on the Pleasant Valley road to the west of Rochdale the sup- 

 posedly Potsdam beds strike n. 42° e. and dip 60° to the southeast; 

 at Rochdale in the road near the mill site, the strike is n. 40° e. and 

 the dip 55° southeast; at the conglomerate ledge on the Titus place 

 Approximately n. 43° e. and 58° s.e. ; at the eastern margin east 

 of Tompkins's n. 28° e. and 35° southeast; north of the Spacken- 

 kill road in the woods near the old barn n. 53° e. and 42° s.e.; at 

 the New Hamburg tunnel about n. 60° e. and 30° s.e. 



Structural features. The presence of an erosion interval 

 between the Trenton beds and the Beekmantown is conclusively 

 shown by the relationships at Rochdale. The Beekmantown is 

 separated from the Trenton by a heavy conglomeratic layer, and the 

 fauna and lithologic character of the two strata are markedly dif- 

 ferent. The general uniformity of dip shows a '' deceptive uncon- 

 formity " or " disconformity." ^ From the apparent thickness of 

 the Beekmantown at Rochdale, it would seem that this formation 

 was not extensively eroded here. 



The limestones of this central strip rest against the slates on the 

 west by overthrust. This is best shown at the north end of the 

 New Hamburg tunnel (see plate 9). The occurrence's of the 

 Potsdam along this western margin is also evidence of it. Fre- 

 quent slips along and across the strike within the limestone are 

 probably present. 



The slates along the eastern margin of the strip may be at places 

 in conformable relationship with the limestone. In other cases such 

 is almost certainly not the case. 



Metamorphism and alteration. The strata composing this 

 strip are all visibly altered. Fossils have usually been greatly 

 obscured. The Beekmantown shows the metamorphism most. 

 Fossils in it are recognized or identified usually with difficulty 

 although they sometimes weather out with distinctness. The Tren- 

 ton beds are usually somewhat crystalline, but fossils are preserved 

 in them in better condition than in the Beekmantown. 



1 Professor A. W. Grabau. Science, n. s., 22 :5S4- 



