DEVONIAN. 307 



Proceeding with the description of the rocks, Hall says regarding the Genesee 

 shale : 



Superimposed upon the TuUy limestone, or, in its absence, resting upon the Moscow shale, 

 we find a great development of argillaceous fissile black slate. Where its edges only are exposed 

 it withstands the weather for a great length of time and often presents mural banks in the 

 ravines, river courses, and upon the shores of lakes. Where the surface of the strata are exposed it 

 rapidly exfoliates in thin even laminae. On disintegration, it is often stained with iron, owing 

 to decomposition of pyrites ; but in many instances and the greater number of localities it retains 

 its deep black color. In this it is distinguished from some beds of black slate in higher situa- 

 tions, which always become stained with hydrate of iron on their edges and upon the surface 

 of the laminae. 



In color and general characters it greatly resembles the Marcellus shale; and aside from 

 position, it would be difficult to distinguish the two in the absence of fossils. * * * 



In lithological character this rock is entirely uniform throughout the district, presenting 

 itself upon the margin of Cayuga Lake and upon Lake Erie, having the same deep black color 

 and laminated slaty structure. Neither is there any change in its organic remains; the same 

 forms, and many of them almost equally abundant, are found throughout its entire extent. 

 The greater portion of this rock is destitute of fossil remains, and it is only toward the upper 

 part that they occur. 



Under the heading "Portage or Nunda group" HaU^®'^ states: 



This group presents an extensive development of shale, shales and flagstones, and finally 

 some thick-bedded sandstone toward its upper part. Like aU the other mechanical deposits of 

 the system, as they appear in New York, it is extremely variable in character at different 

 and distant points. * * * 



From its superior development along the banks of the Genesee River in the district for- 

 merly included in the town of Nunda, now Portage, it has received that name to distinguish it 

 from the higher rocks, which possess some differences in lithological characters but a more 

 striking dissimilarity in organic remains. 



Hall distinguished the Cashaqua shale, Gardeau shale and flagstones, and Port- 

 age sandstone as formations of his "Portage or Nunda group" and described then- 

 characters in detail in the chapter devoted to that group. Clarke ''' has thus 

 summarized Hall's account: 



The succession of strata exhibited in a series of beautiful exposures along the course of the 

 Genesee River was described in some detail by Prof. James HaU in the fourth "Annual report 

 of the survey of the fourth geological district of New York" (1840). 



The beds which lie directly above what was there termed the "upper black shale" (thus 

 contradistinguished from the "lower black shale," the former now known as the Genesee slate 

 and the latter as the Marcellus shale) were designated as the Cashaqua shales, taking their 

 name from the creek entering the Genesee River from the east at Mount Morris. These are 

 olive-green or gray clay shales alternating with bituminous shaly beds and with interbedded 

 thin shales, sandstones, and flags. Upward in the rock series there is in general a gradual 

 increase in the amount of arenaceous sediment, the sands and flags becoming increasingly pre- 

 dominant and the shale beds themselves more arenaceous. While the transition to these beds 

 from those below is in all respects an easy one, yet there is a possibility of distinguishing them 

 in the mass, and hence the upper beds were termed the "Gardeau or Lower Fucoidal group;" 

 the adjective term referring to the abundance of the so-called Fucoides graphica over the 

 lower surfaces of the flags. A thick mass of heavy-bedded greenish or gray feldspathic and 

 quartz sandstones above the Gardeau flags, representing the culmination of arenaceous sedi- 



