GEOLOGY OF SARATOGA SPRINGS AND VICINITY 47 



Since the actual summit nowhere shows, we are somewhat in 

 doubt as to the actual thickness of the formation and as to what 

 directly overlies. The many drilled wells to the south of Saratoga 

 aid in supplying- the information. L'nderneath the Canajoharie shale 

 these wells show an average thickness of from 40 to 45 feet of 

 alternating shale and limestone, beneath which comes ^=^ to 40 feet 

 of limestone and then the Little Falls. The limestone is Amster- 

 dam. Whether any of the alternating shale and limestone is also 

 to be classed as Amsterdam we do not know. We have seen but 

 one exposure of the horizon, in a cut by the roadside a mile east 

 of Rock City Falls, an exposure so poor as really not to merit the 

 name. The fauna was Trenton, not Amsterdam ; but the exposure 

 may not have been basal. We incline to the opinion that this al- 

 ternating shale and limestone is all Trenton, and that the thickness 

 of the Amsterdam is from 40 to 60 feet, varying with the irregu- 

 larity of the floor on which it was deposited. 



The appended list of the more common fossils of the Amsterdam 

 we owe to the courtesy of Dr E. O. Ulrich : 



Solenopora compacta Columnaria halli 



Stictoporella cribrosa Strophomena trentonensis 



Rhinidictya mutabilis Dalmanella rogata 



Phyllodictya varia Dinorthis pectinella 



Eridotrypa minor Bathyurus spiniger 



Arthroclema pulchellum? Leperditia n. sp. 

 Pachydictya acuta var. 



« 



GLENS FALLS ( BASAL TRENTON ) LIMESTONE 



The zone of alternating shale and limestone, some 40 feet thick, 

 which overlies the Amsterdam in the well sections and which is 

 not exposed at the surface, is the only zone in the district to which 

 the term Trenton limestone could at all properly be applied, and 

 there is even question as to its propriety here since the shale exceeds 

 the limestone. Outcrops on the Broadalbin quadrangle and in the 

 eastern Mohawk valley, where the sections are exceedingly like 

 those about Saratoga, leave little doubt that this zone is of lower 

 Trenton age. On the Broadalbin quadrangle Miller has mapped 

 this zone with the Amsterdam, and separate from the overlying 

 Canajoharie shale. The Canajoharie shale, however, is of Tren- 

 ton age, and it seems to us more fitting to group the basal zone 

 which contains the thin limestone bands with this, as its basal por- 

 tion, rather than with the Amsterdam, which we regard as of Black 



