360 NEW YORK STATE MUSEUM 



summit, or any marked pause in sedimentation. Prof. N. H. 

 Winchell has long held, and has recently reiterated the view, that 

 the typical Potsdam at Potsdam is much older than the upper, 

 white, less indurated beds, and he classes it in the middle Cam- 

 bric and correlates it with a portion of the Keweenawan of the 

 upper lake region.^ As above indicated, the writer's judgment is 

 that any present attempt to divide the formation on the basis 

 of age is premature and has but slender basis of fact, consider- 

 ing the lack of all evidence from fossils. 



As has been shown by many observers, the transition from 

 Potsdam to Beekinantown sedimentation is not a sharp one but 

 through a series of passage beds. Near the summit of the former, 

 thin beds of gray dolomite make their appearance, interbanded 

 with the soft, white sandstones which prevail there, increase in 

 frequency till they constitute half the mass of the rock, and 

 finally prevail and cut out the sandstones altogether. The sand- 

 stone layers are characteristically Potsdam in appearance, and 

 the dolomites as characteristically Beekmantown. There is no 

 mixing of materials but rather a rapid alternation of two con- 

 trasted sets of deposition conditions. Walcott has measured a 

 thickness of 25 feet of such passage beds along the Ohateaugay 

 river and 70 feet near Whitehall. In the writer's judgment, the 

 latter is much nearer the usual figure than the former. These 

 beds are exposed at many localities along the northern border 

 of the region, but seldom suitably for measurement of thickness. 

 They seem usually of considerable bulk. 



Beekmantown (Calciferous) formation. Just as in the case of 

 the Potsdam beneath, the Beekmantown formation is thickest on 

 the northeast margin of the Adirondack region and thins out to 

 the west and south, though the thinning is less rapid, so that the 

 formation extends much beyond the limits of the Potsdam, being 

 lacking only on the west side of the region. The type locality is 

 at Beekmantown, Clinton co., where the formation is very fossil- 

 iferous, but where the section is quite incomplete; in fact, there 

 is no one locality in Clinton county where anything like a com- 

 plete section of the formation can be obtained. 



^Am. Geol. April 1903, p.246-49. 



