EEVLEW OF GEOLOGICAL STRUCTURE. 9 



ent bands of limestone, which would certainly -have shared any change pro- 

 duced by acid water in the beds above and below them. 



THE WATKE-LIME GROUP. 



Professor James Hall has recently made the suggestion that the Water- 

 lime Group should rather be united with the Salina than with the Hel- 

 derberg Group, with which it has bem heretofore associated. There is, 

 perhaps, no good reason why this formation should ever have been 

 united with the Helderberg, as they have almost no fossils in common, 

 and they occupy for the most part different areas, still there is apparently 

 no better reason for grouping it with the Salina. It is the product of an 

 epoch of submergence which followed the Salina period when the Salina 

 lake was replaced by, or expanded, to form a water basin of much greater 

 area, and in which the water, although still impure, both from excess of 

 saline matter and of clay, was such as to permit its being inhabited by 

 great numbers of a few kinds of moUusks and crustaceans. 



DEVONIAN SYSTEM. 

 THE 0RI8KANY SANDSTONE. 



In New York the Oriskany varies from less than one foot to thirty feet 

 in thickness ; and is a white, or yellowish, rather coarse sandstone, 

 traceable along a narrow line of outcrop from the Hudson to Lake Erie. 

 It is succeeded above in Eastern New York by the Cauda Galli grit, an 

 arenaceous shale full of the fucoid Spirophyton, from which it takes its 

 name. A thin layer of calcareous sandstone about four feet in thickness 

 rests on this in Albany county, to which the name Schoharie Grit has 

 been given. On the eastern margin of the continent, as well as in the 

 south and west, all this sheet of mechanical sediments is generally rep- 

 resented by limestones. In Canada the Oriskany Group consists of much 

 the same materials as in New York, but is more chprty. 



In New York the fossils of the Oriskany are distinct from those of the 

 Helderberg rocks below, as well as from those of Cauda Galli and the Scho- 

 harie grits above, but in Canada West the most characteristic Oriskany 

 species, such as Spirifera arenosa, S. arrecta, Rensselasria ovoides and Avicula 

 arenosa, are found mingled with Favosites Gothlandica, Zaphrentis prolifica, 

 Conoeardium trigonale, Platyceras nodosum, and many other well known 

 Corniferous fossils. These facts, joined to the entire absence of Upper 

 Silurian species, seem to prove the Orisicany to be much more closely 

 allied to the Devonian than to the Silurian system. 



It is evident that the Oriskany sandstone is the record of a marked 

 change in the physical condition of the region where it occurs, viz. : the 



