122 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



of soft mud derived by erosion of contemporaneous sediments, cast on the beach at times of 

 rough water and flattened and squeezed out by the subsequent pressure and consolidation of 

 the superimposed sand deposits. 



"The upper portion of the formation has frequent beds of irregular laminated sandstone, 

 with partings of greenish arenaceous shale. The shale surfaces are covered with fucoids and 

 worm trails. Pebbles of shale and dolomite, which were hardened before the time of their 

 entombment, are found embedded in the sandstone layers, and their disintegration causes cavi- 

 ties to form in the layers containing them. The dolomite pebbles become more abundant toward 

 the upper horizons. In the upper levels frequent beds are composed of nicely rounded grains of 

 clear quartz with a Httle cement that crumble to a sugary powder under the hammer. Rounded 

 grains of quartz of a sUghtly larger size occasionally cover the upper surface of a layer of finer- 

 grained sandstone, and, being without cement, they stand out in rehef above the surface with 

 an appearance of having been sprinkled from a pepper pot." 



The thickness of the Potsdam in Clinton County is unknown. The thickest mea,sured 

 section is that in the Ausable Chasm, but the section there is compUcated by faulting and is by 

 no means complete, aU the basal portion being lacking. Walcott's measurement gives 350 

 feet, and Van Ingen's " at least 455 feet" as the thickness here. In the MorrisonvUle weU, with 

 the driU resting at 1,250 feet in the Potsdam sandstone, at least 750 feet of the formation had 

 been drilled through, and the bottom samples were of clear, glassy quartz sand, with no trace 

 of the feldspars which characterize the basal portion, indicating that it had not been reached. 

 From this record alone it seems perfectly safe to say that the formation has a thickness consid- 

 erably in excess of that amount in Clinton County. The writer's estimate, based on the broad 

 belt of outcrop in the northern part of the county, assigns a minimum thickness of at least 1,000 

 feet to the formation, with a Ukehhood that it is considerably in excess of that amount. 

 ********* 



The paleontologic and stratigraphic work of Walcott and Van Ingen has shown, that the 

 upper portion of the formation, through a thickness of some 350 feet, carries a sparse Upper 

 Cambric fauna. With the exception of a few supposed tracks, of uncertain nature, no fossils 

 have so far been found in aU the remainder of the formation, and there is therefore an utter 

 lack of paleontologic evidence as to its age, and the possibility that the lower portion may be 

 older than the UppeB Cambric must be conceded. But it seems to the writer that, fossil evi- 

 dence being lacking, the formation as it occurs in New York is not susceptible of subdivision. 

 The basal rocks grade into those of the middle division, as do those into the upper, and there 

 is no marked structural break at any horizon which would warrant the assumption of any great 

 difference in age between base and 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 

 Cambric 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, considering the lack of all 

 evidence from fossils. 



■ As has been shown by many observers, the transition from Potsdam to Beekmantown 

 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 sandstone layers are char- 

 acteristically Potsdam in appearance, and the dolomites as characteristically Beekmantown. 

 There is no mixing of materials but rather a rapid alternation of two contrasted sets of depo- 

 sition conditions. Walcott has measured a thickness of 25 feet of such passage beds along the 

 Chateaugay River and 70 feet near WhitehaJl. In the writer's judgment, the latter is much 



