168 HELDERBERG DIVISION. 



die eastern border of Madison county. It is twenty or twenty-five feet thick in Cherry- 

 valley. Where the streams from the south in the Helderberg range open into the valley 

 of the Mohawk, this rock is usually exposed on the east and west sides, as at Schoharie 

 and Cobleskill creeks. 



s derived from this limestone. So far as the disintegrating mass is concerned, the soil 

 is of an excellent quality ; hut the rock is too limited to exert important effects by itself. 



Thickness. Its maximum thickness is at the base of the Helderberg, where it is seventy 

 or eighty feet thick : at Schoharie, it is sixty feet thick. It is sixty or seventy feet at Be- 

 er. ill's mountain; thence, from the Helderberg, or near New-Scotland, it continually 

 diminishes in thickness to the west. 



$ 4. Encrinal limestone (PI. xx. Sec. 5, 6). 



This rock may be considered by itself, or it may be regarded as the terminal portion of 

 the preceding rock. It differs from the preceding very clearly in its high crystalline 

 structure and light grey color ; but a green matter, or a thin stratum of that peculiar shale 

 derived from the same sources as that of the delthyris rock, appears between its layers, 

 showing that both rocks were derived from one common origin. In composition, it is 

 •liially a pure carbonate of lime crystallized throughout: it is remarkable for the stems 

 of encrinites, which, from something peculiar to these organic bodies, give the rock fre- 

 quently a reddish tint. 



I r as a marble. It is proper to place this mass among the marbles, as it receives a 

 good polish, and is used to some extent for mantle pieces and jambs. It is not, however, a 

 profitable rock for this purpose. It is a strong durable stone, less subject to decomposition 

 than the delthyris shale. 



Thickness. Its maximum thickness is twenty-five feet. It is scarcely ten feet thick at 

 Ni v. Scotland. 



Extent. It accompanies the Delthyris shale as far west as Sharon springs, where it dis- 

 appears ; at least the writer did not observe it at Cherryvalley, where the Delthyris shale 

 is not less than twenty feet thick, and where it onght, if continued, to appear superimposed 

 upon the shale. Its agricultural characters are unimportant. Its geological relations are 

 also scarcely desen ing of notice. Above it is the Oriskany sandstone, when that is present. 

 The pelvis of a large encrinite, of a mammillary shape, or rather shield-form, and nearly 

 two inches in diameter, is considered as its most characteristic fossil. For other relations, 

 see PI. XX. sections 1, 2 and 5. 



§5. Oriskany sandstone (PI. xx. Sec. 1). 



Among the many interesting rocks of New- York, this, which is placed at the bead of 

 the section, is om- that has always excited its full share of attention. It is not because 

 it is economically important, or of great thickness, but rather on account of the number 



