358 NEW YORK STATE MUSEUM 



While these differences are by no means so obvious as those 

 which serve to separate the basal beds from the remainder of 

 the formation, they are sufficiently distinct to enable one to 

 recognize the horizon dealt with in good exposures, provided 

 the extension of van Ingen's work over a larger area shows 

 them to be persistent. The writer's work in the district has 

 been mainly that of hurried areal mapping, and' is not there- 

 fore of sufficiently detailed character to enable him to express 

 an opinion of any value on this work of van Ingen's, though, 

 so far as it goes, he is disposed to coincide. Precisely the same 

 differences which van Ingen notes have been observed, but the 

 work was not done in sufficient detail to warrant publication. 

 In addiition it may be stated that frequent pebbly beds occur 

 throughout the formation, in which the pebbles are almost ex- 

 clusively of white quartz, with a tendency to concentration on 

 the upper surfaces of layers which are otherwise of pure sand, 

 instead of being disseminated through the layer. Such pebbly 

 horizons seem much more characteristic of the middle division 

 than of the upper. 



The thickness of the P'otsdara in Clinton county is unknown. 

 The thickest measured section is that in the Ausable chasm, but 

 the section there is complicated by faulting and is by no means 

 complete, all the basal portion being lacking. Walcott's meas- 

 urement gives 350 feet, and van Ingen's '' at least 455 feet " 

 as the thickness here. In the Morrisonville well, with the drill- 

 resting at 1250 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 per- 

 fectly safe to say that the formation has a thickness consider- 

 ably 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 likelihood that it is consider- 

 ably in excess of that amount. 



^19th An. Rep't State Geol. p.r69. 



