THE SILURIAN. 23 



Helderberg Limestones. There are thus two depressions separated 

 by an intervening elevation. 



In Newfoundland the Silurian rocks occur in a narrow troutrh 

 extending through tlie centre of the island, and, so far as can be ascer- 

 tained from the Reports of the Survey of Newfoundland, are not 

 dissimilar from the exposures in Nova Scotia. 



In the latter province the great limestones are, as stated in the 

 earlier editions, absent or represented by comparatively insignificant 

 and impure bands. Shales with some sandy beds (Lower Arisaig 

 beds of previous papers) represent the Clinton, and contain GraptoUthus 

 dintonensis ; coarse impure limestone and shale (New Canaan beds of 

 previous papers) correspond to the Niagara, holding characteristic 

 corals of this age, and shaly beds with thin layers of limestone (Upper 

 Arisaig of previous papers) represent the Helderberg. In Nova 

 Scotia these occur in the New Canaan, Arisaig, and Pictou districts, 

 and their charactei-s correspond to those seen in Newfoundland, New 

 Brunswick, and Maine. In the Cobequid Mountains of Nova Scotia, 

 however, and in New Brunswick, these beds, especially in their upper 

 part, show great contemporaneous emissions of igneous rock. These 

 are partly felsitic and partly doleritic and amygdaloidal. They 

 correspond in age with those isolated igneous masses of the plain of 

 the St Lawrence to which the Montreal and Beloeil Mountains belong. 



In proceeding to the west and north, the Helderberg Limestones 

 appear in great force at Cape Bon Ami in Northern New Brunswick, 

 where they are rich in fossils and associated with beds of trap. Both 

 limestones are largely developed in Bonaventure and Gaspe, and the 

 lower member in the Island of Anticosti, so that here, as in previous 

 periods, the area of the Gulf of St Lawrence corresponds with the 

 interior plateau rather than with the coastal region. In some respects, 

 indeed, this area presents an exaggeration of the interior conditions, 

 since in Anticosti there is apparently a gradual passage from the 

 limestones of the Hudson River group to those of the Clinton, with- 

 out the intervention of sandstones similar to the Oneida and Medina 

 of New York and Ontario. In so far as I am aware there is also an 

 absence of beds representing that condition of deserts and salt lagoons 

 represented by the Salina or Onondago salt-group. In this last 

 respect, as in so many others, the conditions of the eastern districts 

 of America conform to those of Europe, and not to those of the 

 interior plateau of America. 



In America as in England the Silurian of the maritime districts is 

 unconformable to the Cambro-Silurian, though this docs not hold in 

 Anticosti or in the inland region. 



