THE LOWER AND UPPER SILURIAN AGES. 61 



parts of the Atlantic, on tlie European side, as we 

 have long known that they exist at less depths on the 

 American side; and these same researches, with the 

 soundings on the American banks, show that sand 

 and gravel may be deposited not merely on shallows, 

 but in the depths of the ocean, provided that these 

 depths are pervaded by cold and heavy currents 

 capable of eroding the bottom, and of moving coarse 

 material. The Quebec group in Canada and the 

 United States, and the metalliferous Lower Silurian 

 rocks of Nova Scotia and Newfoundland, destitute of 

 great marine limestones and coral reefs, evidently 

 represent deep and cold-water areas on the border 

 of the Atlantic plateau. 



At a later period, the beginning of the Upper Si- 

 lurian, the richly fossiliferous and exceptional deposits 

 of the Island of Anticosti, formed in the deep 

 hollow of the Gulf of St. Laurence, show that when 

 the plateau had become shallowed up by deposition 

 and elevation, and converted into desolate sandbanks, 

 the area of abundant life was transferred to the still 

 deep Atlantic basin and its bordering bays, in which 

 the forms of Lower Silurian life continued to exist 

 until they were mixed up with those of the Upper 

 Silurian. 



If we turn now to these latter rocks, and ir |uire 

 as to their conditions on our two great plateaus, we 

 shall find a repetition of changes similar to those which 

 occurred in the times preceding. The sandy shallows 

 of the earlier part of this period give place to wide 



