572 THE UPPER SILURIAN. 



either but slightly developed or obscured by imperfect exposures, 

 and on these rest the Lower Devonian slates and iron ore of 

 Nictaux, already described. 



Regarding the above as the most typical and most thoroughly 

 explored portions of the Upper Silurian of Nova Scotia, it is important 

 to attain to as correct notions as possible as to their equivalency 

 with the beds of that system elsewhere. In estimating this, we 

 must bear in mind the fact that they belong to the Eastern or 

 Atlantic slope of America, in which the Ujjper Silurian rocks 

 are not only more altered by heat and chemical agents than in 

 the great central plain west of the Alleghauies, but appear to have 

 diflfered in the original character of the deposits. These Avould seem 

 to have been more aflPected by local differences of deposition, so as 

 to produce great diversities of mineral character within limited 

 distances. They seem also on the whole to have been more argil- 

 laceous and less calcareous. These considerations may serve to 

 account for the apparant absence of the great Niagara formation, 

 the equivalent of the English Wenlock, from the Arisaig section, 

 while the Clinton is greatly developed ; and the Niagara formation, 

 under a peculiar modification, occurs in considerable thickness 

 at New Canaan and Kentville. Beyond the limits of Nova Scotia, 

 the Upper Silurian of Southern New Brunswick and of the State 

 of Maine presents much resemblance both in its mineral character 

 and fossils to the Arisaig group in Nova Scotia. On the other 

 hand, in Northern New Brunswick and Gaspe, beyond the great 

 Lower Silurian belt of Northern New Brunswick, the Upper Silurian 

 becomes more calcareous, and differs much in Its fossils from the 

 Upper Silurian of Nova Scotia. The Island of Anticosti presents 

 another development of the lower part of the Upper Silurian not 

 hitherto recognised in Nova Scotia. 



In the presence of so great local diversity, it seems chimerical to 

 compare our Upper Silurian either with the fine and regular series of 

 New York and Upper Canada (Ontario) or with the English series. 

 It must be admitted, however, that, in a general way, the Nova Scotia 

 Upper Silurian presents in its fossils characters in some respects inter- 

 mediate between the American and Evu'opean series, and therefore 

 comparable with either or both. As the general result of the facts 

 already stated, in their bearing on these questions, I may state the 

 following conclusions: — (1.) The Upper Arisaig and Nictaux series 

 may be regarded as on the horizon of the Lower Helderberg of 

 New York and the Ludlow of England, though with some older 

 forms among their fossils. (2.) The New Canaan beds are probably 



