334 G. H. CHADWICK STRATIGRAPHY OF NEW YORK CLINTON 



formable contact of the marine Clinton beds on those beneath.^ The 

 barren green shale at Eochester belongs, therefore, helotu this disconfor- 

 mity and with the Thorold sandstone. Thongh the line between these is 

 ^^sharp/^ in the sense of there ])eing no extended gradation, they plainly 

 blend into a single group at Rochester. 



Correlations Eastward 



It was originally intended to end this paper at the Oswego Eiver, in 

 the belief that the stratigraphy to the east was too complicated or obscure 

 to be solved with present data. But an intensive study of the eastern 

 sections dispels so much of their obscurity that a second portion of the 

 chart (figure 2) has been prepared to illustrate how the correlations can 

 be (very tentatively) carried eastward, even to the limit of outcrop of 

 this group in Xew York, and thus a groundwork of hypothesis laid for 

 future field study of these eastern sections. 



In this portion of the chart the greater thickness of the Clinton has 

 compelled a smaller vertical scale, and it has also been necessary to show 

 the formations above the Eochester shale, since the Eochester soon ceases 

 to appear in the more easterly sections. In its absence the line of uncon- 

 formit}' at the summit of the Clinton is taken as the datum until this 

 line begins to bevel sharply down across the Clinton strata. 



The chart recommences with the South Granby well section, which is 

 repeated on this plate. The next section, an incomplete one, is at Brew- 

 erton, which lies at about the middle of the width of the Clinton belt and 

 therefore presumably midway between base and summit. The records 

 of the Sja'acuse wells (9:24-25; 11:8), located on the same meridian, 

 since the combined Eochester and Lockport can here scarcely exceed 200 

 feet, would indicate a minimum of 225 to 230 feet of Clinton strata here- 



3 This appears to be an error. According to Dr. M, Y. Williams ( private communica- 

 tion), Logan's account was based on that of Alexander Murray in the First Annual Re- 

 port of the Canadian Sui-vey (1843), p. 75. Both accounts give 10 feet of the gray 

 (Thorold) sandstone overlaid by 4 feet of a '"bluish-green argillaceous shale" containing 

 Arthrophycus, especially near its contact with the sandstone, and followed in turn by G 

 feet of calcareous beds before the Pentamerus layer is reached. Neither Doctor Williams 

 nor Doctor Kindle find any trace of this shale in the exposures seen by them or in the 

 records of the Welland Canal engineers. Doctor Kindle's field notes (transcript kindly 

 furnished the writer) show the Pentamerus layer resting directly on but 8 feet of Thorold 

 sandstone. Although this section was clearly made at a different point from Murray's, 

 the discrepancy (of 12 feet of strata) seems too great to be accounted for even by the 

 known disconformity. 



There may be a question as to the identity of the "gray baud" at Rochester with the 

 Thorold sandstone. According to Hall (Annual Report for 1837, p. 297), the strati- 

 graphic continuity is interrupted in the Lockport region. Observers agTee that the true 

 Thorold is closely linked to the subjacent red beds, Avhereas the converse is true of the 

 gray sandstone at Rochester. This matter requires investigation. 



