418 CAMBRIAN AND SILURIAN IN NORTH AMERICA. [XV. 



Upper Cambrian, and the Upper Llandovery or May Hill 

 sandstone, the base of his Silurian. These two contiguous 

 though discordant formations, in fact, exhibit a mingling of the 

 forms of the second and third faunas. It is, however, to be 

 noted that the Middle Silurian thus denned is by no means 

 the equivalent of that of Mr. Billings, who has given the name, 

 not to beds of passage, but to a group well denned both strat- 

 igraphically and paleontologically equivalent to the Upper 

 Llandovery and the Wenlock of England, or, in other words, 

 to the fossiliferous strata between the top of the Hudson River 

 shales and the summit of the Niagara limestone (including the 

 Guelph) ; thus taking the lower half of the true Silurian or the 

 Upper Silurian of Murchison. That the group of strata com- 

 prised under this latter name (the third fauna of Barrande) 

 really includes two very distinct faunas, was long since shown 

 by Hall, the break between the two being marked in New 

 York and Ontario by the interposition of the non-fossiliferous 

 Onondaga or Salina group. This series of strata, in some 

 parts 1,000 feet or more in thickness, consists of red and green 

 magnesian marls with rock-salt and gypsum, overlaid by a great 

 mass of magnesian limestones, the whole having been deposited 

 in a vast mediterranean basin which extended from eastern 

 New York to Ohio. The Water-lime beds, with their peculiar 

 fossils, overlying the Salina group, consist of a magnesian lime- 

 stone lithologically related to the rocks below, and represent 

 the first invasion of life into the former dead sea, which was 

 followed by the great deposit of non-magnesian limestones of 

 the Lower Helderberg group. These, which attain a great 

 thickness to the eastward, make up, with the Oriskany sand- 

 stone, a fourth palaeozoic division, the equivalent of the Ludlow 

 of England. In Gaspe" a sandstone formation, without any ap- 

 parent unconformity, connects the Oriskany with the great mass 

 of Devonian sandstones ; but in New York and in Ontario evi- 

 dences of an interruption in the process of deposition are seen 

 in the erosion of the Oriskany previous to the deposition of the 

 Corniferous limestone, which there forms the base of the De- 

 vonian or the Erie division of the New York system, extending 



