296 NEW YORK STATE MUSEUM 



Siluric around the southern margin of this barrier, hence exclud- 

 ing these in large measure from the outcrops now exposed along 

 the northern and eastern edges of the Helderberg escarpment. 



The key to the recognition of these physiographic differences 

 east and west was found first in the stratigraphic value of the 

 formation in eastern New York termed by the early geologists 

 the " Coralline limestone " and accepted by them and their suc- 

 cessors as such eastern representative of the Upper Siluric Niagara 

 or Lockport dolomitic limestone of western New York. We term 

 this formation at the present time, or such part of it as was called 

 by Hall, who described its fossils, Coralline limestone, the Cobles- 

 .kill dolomite. 



The very refined study of this formation made by C. A. Hart- 

 nagel and published in these reports has shown not only the con- 

 tinuity of this unit across the State from west (Buffalo) to east, 

 interrupted from Schoharie to Rondout or along the north edge 

 of the Helderberg, but also demonstrated its position at the top 

 of the Salina formation, hence of far later age than the Niagaran. 

 With this well determined fact in hand this formation, heretofore 

 used as a bench mark for the assignment of the formations in 

 eastern New York above and below it, again serves a similar pur- 

 pose and necessarily involves important modifications in correlation. 

 Following the clues herein suggested, Mr Hartnagel has recently 

 given attention to the stratigraphic value of the Shawangunk grit 

 which constitutes as a ^eavy sheet of arenaceous deposits all except 

 the basal parts of the Shawangunk mountains in Ulster county 

 and extends south through western Orange county into New Jersey. 



This extensive series of arenaceous deposits has been, from the 

 earliest classification of the formations, uniformly interpreted as of 

 the age of the Oneida conglomerate of central New York, but 

 Hartnagel's researches indicate with probability that in its typical 

 localities this Oneida conglomerate is a local phase of Medina 

 sedimentation and lies within the recognized upper limits of that 

 formation. In the Skunnemunk region the Shawangunk grit is 

 separated from the Cobleskill limestone by a series of formations 

 of upper Salina age which in their extension from Ulster county 

 to New Jersey vary considerably in thickness, lithologic character 

 and fossil contents. 



The details of these later correlations are published in Mr 

 Hartnagel's article appearing in another part of this bulletin. The 



