GEOLOGY OF OGDENSBURG 35 



to commend it. But additional work needs to be done before this 

 can be put forward as anything more than a working hypothesis. 

 As such, however, it needs to be kept in mind by future workers 

 in the field. 



ORDOVICIAN FORMATIONS 



Tribes Hill formation 



General statement. Blue, sandy limestones containing as fossils 

 "Pleurotomaria hunterensis" and cystid plates, iden- 

 tical with forms found in the Tribes Hill formation of the Mohawk 

 valley, were found, with a thickness of some 30 feet, in the Thou- 

 sand Islands region, and correlated with the Tribes Hill. We were 

 accordingly on the lookout for this formation, while the work on 

 the Brier Hill sheet was in progress, and were much surprised to 

 find little or no trace of it in the excellent sections east from Morris- 

 town, while we found instead the considerable thickness of sandy 

 beds containing the Heuvelton sandstone, which have just been 

 described. The small thickness of gray, calcareous beds, no. 7 of 

 the section described on page 31, suggested Tribes Hill, but no 

 fossils were found, and the zone was very thin. Scraps of lime- 

 stone were also found in poor exposures in the heavily drift- 

 covered portion of the Ogdensburg sheet, also with no fossils. 

 Hence we came to the conclusion that the formation had nearly or 

 quite disappeared in the interval between Theresa and Morristown. 

 In his work on the Canton quadrangle, however, Professor Chad- 

 wick found a considerable thickness of beds at this horizon, from 

 which he collected fossils, and for which he suggested a Tribes 

 Hill age. His fossils were determined by Doctor Ruedemann, and 

 subsequently Doctor Ulrich also examined a portion of his collec- 

 tion, and both gentlemen agree in their determination of these beds 

 as of Tribes Hill age. We have therefore mapped a belt of this 

 formation across the Ogdensburg quadrangle, to include the 

 scattered outcrops of limestone. The mapping is, however, of 

 the most perfunctory character since, in that drift-covered belt, 

 no accurate mapping is possible, and geologic boundaries can be 

 delineated only in a most general manner. On the Brier Hill sheet 

 the formation, if present, is so thin and so patchy in distribution 

 that it is impossible to map it on this scale. 



This variable distribution of the formation definitely suggests 

 one of two things : either the Tribes Hill shore line in this part of 

 the State was irregular, setting back into embayments on the 



