386 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Genesee Slate gives us six original Invertebrates, four being Bra- 

 chiopoda, together with a well-defined leaf of grass or sea-weed, with 

 a smooth surface 6 inches long by -jl-th of an inch broad ; Vanuxem 

 also found this grass- like plant in the Portage group. 



Prof. Rogers mentions in the Pennsylvanian continuation of this 

 stratum numerous small and delicate Molluscs, chiefly of Devonian, 

 but some of Carboniferous, genera. It also there contains a terrestrial 

 vegetation generically identical with the Coal-measures. 



Fossils occurrent in Eurojje. — We only know of one, the Orbicula 

 Lodensis, Hall. 



Fossils recurrent in New York. — Five connect this rock with its 

 near relation Marcellus Shale. Two other fossils (Atrypa didyma 

 and A. reticularis) are received from the Clinton, Onondaga-Salt, and 

 other Silurian groups, assisting therefore to establish the connexion 

 between the different parts of the palaeozoic systems. 



Fossils typical. — These are four. They are marked with an * in 

 the above list. 



Portage Group (or Nun da). 



Mineral character. — It is an extensive development of argillaceous 

 and sometimes micaceous shales and flagstones, and, finally, of some 

 thick-bedded sandstones towards its upper part. Like all the me- 

 chanical deposits of this system in New York State, it is very variable 

 at different parts. 



Shales and sandstones compose the whole. In the lower part, 

 these are more equally intermingled, and the sandstone is finer, — while 

 higher up, the sand is coarser, and there is less shale. 



It has been divided into three parts, beginning from below, as 

 follows : — 



a. Cashaqua Shale. — This division is worthy of a separate name, 

 and is called after the Creek Cashaqua. It is there a green, soft, 

 argillaceous rock, which crumbles into tenacious clay. It contains 

 calcareous concretions. 



b. Gardeau Shale and Flagstones. — These, placed above the Ca- 

 shaqua Shales, are a great accumulation of green and black slaty 

 and sandy shales, with thin layers of sandstone. 



c. Portage Sandstones consist of thick-bedded sandstones with 

 little shale. The lower part is of thinner sandy layers with more 

 shale. Its thick-beddedness and the abundance and variety of its 

 fucoids (some vertical) may perhaps justify the separation of this 

 group from the rocks below : but it is only in the State of New York 

 that the Portage group ends with these thick-bedded sandstones ; it 

 is not so in the Western States, Illinois, Indiana, Ohio, &c. In- 

 deed all these palaeozoic rocks, as before said, vary greatly with place. 

 In one locality the line of division among its parts is clear, at another 

 obscure, or very gradual*. 



The materials of the Portage group (Hall, Rep. p. 254) came from 



* Prof. Rogers (Johnston's Atlas) defines the Portage group, as met with in 

 Pennsylvania, to be a rather fine-grained grey sandstone, in thin layers or flags, 

 and parted by thin bands of soft blue shale. 



