70 NEW YORK STATE MUSEUM 



exposed in many localities, as on Bald mountain near Greenwich, 

 about Granville, etc. The name is taken from the fine exposures 

 two miles south of Schodack Landing, N. Y., on the bank of 

 the Hudson river and the belt of these rocks in the town of 

 Schodack, N. Y. 



5 Nassau beds (divisions A— E of Dale's series in Rensselaer 

 county). In Rensselaer county the Olive grit or Bomoseen grit 

 is underlain by a series of alternating reddish and greenish shales 

 and quartzites containing Oldhamia, about 150 to 800 feet thick. 

 This is especially well exposed in the town of Nassau, N. Y. 



6 Diamond Rock quartzite (division G of Dale's Rensselaer 

 series). This division 10 to 40 feet thick and composed of gran- 

 ular quartzite and associated calcareous sandstone, is well exposed 

 in Oakwood cemetery and the " Diamond Rock " in Lansingburgh 

 (North Troy), from which it takes its name. 



7 Troy shales (division H of Dale's Rensselaer series). This 

 formation, which follows upon the Diamond Rock quartzite, con- 

 sists of 25 to 100 feet of colored shales with small beds of cal- 

 careous quartzite. The shale has furnished Oldhamia, a calcareous 

 sandstone bed in the upper part Hyolithes and Hyolithellus. These 

 beds are well exposed at Troy, at the dam in the Poesten kill and 

 other localities. 



8 Zion Hill quartzite (Ferruginous quartzite Dale). This 

 name, taken from Zion hill, Hubbardtown, Vt., where according to 

 Dale the ferruginous quartzite is exposed in a thickness of 70 feet, 

 is proposed here for the sake of completeness. 



On the Schuylerville sheet we find well represented only division 

 A and the limestone and shales of D. We have separated the areas 

 occupied by these two divisions on the map, the boundaries being 

 only approximate on account of the interfolding of the beds. 



The olive grit occupies the southern half of the area. It is easily 

 recognized by the pale brick-red color of the weathered crust that 

 forms on it ; typically it is seen on the many ledges north of Green- 

 wich, but it also appears on all sides of Louse hill and extends to 

 the southern boundary of the Georgian areas. From Greenwich the 

 grit skirts the eastern side of Bald mountain. It is described by^ 

 Dale as follows : 



A greenish, usually olive-colored, very rarely purplish, more or less massivel 

 grit, generally somewhat calcareous, and almost always spangled with veryi 

 minute scales of hematite or graphite. Under the microscope it is seen to 

 consist mainly of more or less angular grains of quartz, with a considerable 



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