888 NEW YORK STATE MUSEUM 



acter of the feldspar grains in this portion may perhaps be thus 

 explained.^ 



Above the basal division the rock becomes a purely quartzose 

 one, and the red color disappears. This middle portion of the 

 formation, as shown in Clinton county, becomes however its base 

 to the west and south, because of the gradual encroachment of'the 

 subsidence in those directions, explaining the lack of arkose in 

 St Lawrence county and in the Mohawk valley. Because of the 

 occurrence, in St Lawrence county, of a quite pure, quartz sand- 

 stone resting on an uneven surface of crystalline rocks, Smyth 

 has argued for a humid climate with rapid weathering at the 

 time ; and that, owing to the resistant character of the underlying 

 rocks, the waves did not act for a sufficiently long time at a given 

 level to plane away the rock floor to an even surface, though the 

 time was sufficiently long to weather and triturate all minerals 

 save the extra resistants quartz.^ The writer quite agrees that a 

 change in climate is probablj^ indicated by the change in the rock 

 character. 



The offshore mud deposits of Potsdam time are nowhere ex- 

 posed to view about the region. Some slight deposit of limestone 

 took place, as shown by Walcott for the district about Saratoga, 

 and by the well records published by Orton for the Oswego county 

 region. The formation tends often to become somewhat calcareous 

 or rather dolomitic, above, and everywhere grades into the over- 

 lying Beekmantown through a series of passage beds, which show 

 rapid alternations of the two sets of contrasting conditions, the 

 one gradually overcoming the other, so that subsidence must have 

 been progressive, and no great time interval could have elapsed 

 between the two deposits. 



The bulk of the Beekmantown formation is composed of sandy 

 dolomites, all very barren of fossils. They seem to the writer to 

 indicate shore conditions and to a considerable extent " salt pan " 

 conditions.^ With the oncoming of Beekmantown time subsidence 



^The writer has long been of the opinion that the early Potsdam climate 

 was an arid one; and recent correspondence with Mr van Ingeu has dis- 

 closed that he is also disposed to hold a similar view and is investigating 

 the matter with the development of new and interesting evidence. 



^N. Y. State Geol. 19th Rep't 1899, p.rlOO-2. 



''Dana, J. D. Man. of Geol. ed. 4, p.l33. 



