GEOLOGY OF SARATOGA SPRINGS AND VICINITY 35 



the next 50 feet sandstone and calcareous sandstone alternate, with 

 an occasional bed of blue, sandy dolomite in addition. The upper 

 100 feet consists of alternations of all three. This we definitely 

 assign to the Theresa formation. The lower 100 feet we assign 

 to the Potsdam, notwithstanding the presence of an occasional 

 dolomite bed. There is never any sharp boundary between the two 

 formations. 



THERESA FORMATION 



Everywhere in New York where the Potsdam sandstone occurs, 

 it is united with the formation next above, usually a dolomite 

 formation, by a series of intermediate or passage beds which con- 

 sist of alternating sandstone, calcareous sandstone and dolomite. 

 These passage beds differ from the sandstone beneath and the 

 dolomite above, are sufficiently thick to require mapping as a 

 separate formation, as such require a name, and we are calling 

 them the Theresa formation.^ The sandstone is in excess in the 

 lower part of the formation, and the dolomite in the upper. Deposi- 

 tion was apparently continuous but physical conditions slowly 

 changed, the sand supply from the Adirondack land slowly dimin- 

 ishing and finally being entirely cut off. Because of this it is 

 impossible sharply to delimit the formation, either at base or sum- 

 mit. In what we here class as upper Potsdam there is an occasional 

 bed of dolomite, and a few stray beds of sandstone run well up 

 into the overlying dolomite formation. Nevertheless there is a 

 sufficient thickness of mingled sandstone and dolomite, about which 

 there could be no possible difference of opinion, to justify its sepa- 

 rateness as a formation. 



In the general region the Theresa formation has a thickness of 

 from 150 to 200 feet. Thus ^liller gives its thickness as 200 feet 

 on the Broadalbin quadrangle, while our measured sections at 

 Whitehall and at Ticonderoga show a thickness of at least 150 feet. 

 There is at least 150 feet of it in the section south of East Galway, 

 on the Saratoga quadrangle, and the full thickness is not there 

 exposed. In all these localities the formation consists of alter- 

 nating beds of hard, vitreous sandstone, weak calcareous sandstone 

 and sandy blue dolomite, with L i n g u 1 e 1 1 a acuminata as 

 its most abundant and characteristic fossil. 



West and northwest of Saratoga, in Greenfield township, the 

 section differs. Instead of a thickness of 150 to 200 feet, the 



' N. Y. .State Mus. P.ul. 145. p. 64-66. 



