436 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



We have in the middle stage (Clinton section) probably the first 

 considerable aggregation of iron-ore in sedimentary rocks. 



As might have been anticipated, the middle stage does not abound 

 in traces of life. For 1000 feet above the Hudson-River Rocks, west 

 of the Oneida Conglomerate, we have a homogeneous argillaceous 

 sandstone totally barren of fossils ; but at about 1 00 feet from the 

 top of Medina Sandstone they begin to appear. 



The supposed marine plants, Palceophyci, &c, become, by their 

 vast assemblages, and sometimes by the beauty of their forms, a pro- 

 minent feature of this stage. 



The Brachiopoda here (Hall, Rep. p. 53) contrast with those of 

 the lower stage, and species are introduced eminently significant of a 

 new phase. 



Species of Orthides are much fewer, and even individuals are rare. 



In the Leptsenoid type we have now for the first time the crenulate- 

 hinged Strophodonta and the remarkable mollusc Chonetes. Atrypce 

 are more numerous here than Leptcena or Orthis, compared with the 

 lower stage ; and they have the smooth, round, or subcylindrical 

 forms which rarely occur in the lower beds. Lastly, we have the 

 genus Pentamerus, which is wholly unknown in the stage below. 

 Three species, P. oblongus, P.fornicatus, and P. ovalis (Hall), are 

 restricted to the Clinton section, but others mount up into Upper 

 Silurian. 



There are no Trilobites in Medina Sandstone, but we meet with 

 twelve in the Clinton group ; of these, seven are typical. One species 

 of the genus Beyrichia appears here, imbedded in iron-ore. 



The Upper' Stage. — The upper stage of this system is constituted 

 best by placing the Niagara section at its base, and by including all 

 the superincumbent strata up to Oriskany Sandstone. All these 

 rocks have comparatively permanent mineral characters. The nume- 

 rous changes of the middle stage have either moderated or ceased 

 altogether. 



The Niagara and its four kindred limestones (Lower Helderberg) 

 consist wholly of argillaceous shales and limestones, the latter in 

 Niagara being sometimes pure in the west, or mixed with magnesia, 

 or white and sparkling with siliceous drops. 



The fossils of the upper stage are, except in parts, abundant, 

 various, elegant, in great measure typical, and gregarious. Upon 

 this subject we refer to the observations on Group F in page 432. 



The Devonian System. 



The establishment of a new system seems to be called for on the 

 completion of the Upper Pentamerus Limestone. The call has been 

 obeyed, and the new system has been named " The Devonian." It 

 extends from Oriskany Sandstone to the summit of the Catskill 

 group, or the Old Red Sandstone. 



The Devonian system is established in New York on the following 

 grounds, among others : — 



1. It is introduced by gritty calcareous sandstones, composed 



