GEOLOGY OF THE NORTHERN ADIRONDACK REGION 303 



Beekmantown, and that they should either be placed with that 

 formation or else considered as distinct from either and given 

 a separate name, " as Fort Cassin, or Philipsburg formation, or 

 any other appropriate name." 1 This seems to the writer not only 

 an eminently proper, but really a necessary procedure. The thick- 

 ness and importance of this group, consisting of the upper 220 

 feet of division D and the whole of E, is such as definitely to war- 

 rant its separate mapping in the Champlain region, and the 

 writer proposes the name " Cassin formation " for it, to make 

 Whitfield's suggestion more precise and definite. The question 

 as to whether the rocks involved are to be classed with the Chazy 

 or with the Beekmantown, or with neither, is not at issue in the 

 giving of the name, the point made being simply that we have here 

 several hundred feet of limestone of definite lithologic and paleon- 

 tologic character, whose definiteness and importance would seem 

 to warrant its separate recognition and mapping as a substage. 

 In the type section, at Beekmantown, the rocks of this upper 

 horizon are not exposed. The matter will be reverted to on a 

 subsequent page in considering the subdivisions of the Chazy 

 limestone. 



When- the Beekmantown formation is followed to the south and 

 around into the Mohawk valley, it is found to be much thinner 

 than in the Champlain region, and the upper portion, that which 

 has been separated above as the Cassin formation, is wholly 

 wanting. Prosser has carefully measured a number of sections in 

 this region, but measurements of the full thickness can rarely be 

 made because the formation is seldom cut through to its base, 

 and because it overlaps on the Precambric, thus not presenting 

 its full thickness close to the Precambric border. At Little 

 Falls the formation is 456 feet thick. The deep Avells at Ilion 

 and Utica, a few miles farther west and at a somewhat greater 

 distance from the Precambric edge, show a somewhat, but not 

 greatly increased thickness. Eastward from Little Falls the sec- 

 tions in the valley do not get down to the base of the formation, 

 except at Spraker, where Prosser measured a thickness of 500 

 feet, with the summit not exposed, but the thickness does not vary 

 greatly apparently, though showing some diminution east of 



l Am. Mus. Nat Hist. Bui. 3 :2T-28. 



