DESCRIPTION OF THE FORMATIONS F 2 . 55 



which it and the strata adjoining it may easily be recog- 

 nized." 



It occurs however near the top of the Variegated shale, 

 not of the Gray shale, in which horizon no such bed exists 

 in Perry county. 



A great error in stating the thickness of the Variegated 

 marls also occurs on the same page. They are said to be 

 " probably not more than 100 feet thick," near Tuscarora 

 mountain. They are 700, as stated above. 



Onondaga gray shales. 



In regard to the uppermost part of the New York Onon- 

 daga deposit the comparison made above is also to a great 

 extent true. The description given of it is largely appli- 

 cable to the upper part of these Pennsylvanian shales, the 

 Scalent gray marls of Rogers. Vanuxem says, (p. 99 :) 

 "The great mass of the deposit consists of rather soft yel- 

 lowish or drab and brownish colored shale and slate both 

 argillaceous and calcareous." So in Perry county, though 

 seldom well exposed, the mass is of this kind. It contains 

 " argillaceous and calcareous slaty and more compact masses 

 which are hard." 



But there is no evidence in Perry county of the presence 

 of those concretions of gypsum which characterize the upper 

 part of the Onondaga in New York, and which together 

 with its brine springs render it the most valuable rock in 

 the State. The gray shales contain no valuable mineral ex- 

 cept the lime which enters largely into their composition 

 and which gives the soil derived from their disintegration 

 a value almost equal to that of the adjoining "limestone 

 land." Their area is not large, forming only a narrow fringe 

 between the variegated shale and the base of the Lower 

 Helderberg limestone. Owing to their softness, also, they 

 are seldom exposed. 



The great barrenness of these shales in the matter of 

 fossils prevents the production of satisfactory evidence 

 of age. Only two or three species have rewarded a con- 

 siderable amount of search. Of these the only one that 

 occurs in any quantity is Leper ditia alia, which has been 



