THE ASHOKAN FORMATION 469 



siliferous throughout, except for stipes of fernlike plants Avliich abound 

 on the surfaces of certain sandstone layers. 



This formation was mapped by Prosser as the Sherburne sandstone 

 of the Helderberg region and ma-de continuous with the Sherburne of the 

 type section. Of course, no continuous tracing of these beds from the 

 Chenango Valley to the Helderberg was possible, and the correlation was 

 made on the lithic similarity, the presence of similar plant stipes in both, 

 and the fact that in Ulster, as in Chenango County, the rock of this 

 character directly succeeded the fossiliferous Hamilton beds. Since rocks 

 of somewhat similar character but with a modified Hamilton fauna suc- 

 ceeded the Hamilton of Otsego and Schoharie counties, it appeared that 

 the Sherburne was a continuous formation lying at the base of the Upper 

 Devonic of eastern ISTew York and in part replacing the Tully limestone 

 and Genesee shale of central 'New York. In my summary of the Ulster 

 County section in the Schoharie Guide,^^ I have followed Prosser and 

 used the name Sherburne for these unfossiliferous lower flags, and the 

 same name was used by Berkey and by other writers on the rocks of the 

 bluestone region. Continued study of these formations for a number of 

 years has, however, convinced me that this correlation is not the correct 

 one, and I have adopted for these beds the local name Ashohan formation, 

 from the iVshokan district west of Kingston, New York, where the rock 

 was quarried near, and for the use of the construction of, the Ashokan 

 reservoir of the Catskill water system of New York City. I am satisfied 

 that this formation is a continental phase of the Upper Hamilton and 

 therefore beneath the base of the typical Sherburne sandstone. This is 

 shown by the fact that in the Chenango Valley region the Hamilton, 

 which underlies the Sherburne sandstone, and the Marcellus, which lies 

 beneath it, have a combined thickness of 1,500 feet. At Schoharie, where 

 the equivalent sandstones of the Sherburne carry an Ithaca fauna while 

 still retaining the thickness of the typical Sherburne, the Hamilton is 

 about 1,500 feet thick, while the Marcellus below it has a thickness of 

 180 feet, or about 1,680 feet for both formations. In Ulster County, on 

 the other hand, the marine beds which underlie the Ashokan formation 

 have a thickness of only about 400 or 500 feet. There are perhaps some 

 300 feet of shale between the exposed sandstones and shales and the 

 Onondaga, a part of which must be classed as Marcellus. Farther north, 

 at Clarksville, the Marcellus type of shale is Avell exposed, being 300 feet 

 Thick, while the more sandy Hamilton type is 490 feet and may have been 

 500 feet or more. 



The passage of the marine into the non-marine beds of the Ulster 



N. Y. state Mus. Bull. 75, p. 303. 



