154 THE CANADIAN NATURALIST. [Vol. 



X. 



Source of the Mechanical Deposits. — From the character of 

 the rocks and their distribution in the Niagara period, as seen 

 by glancing at a map of the Palaeozoic Geography of America, 

 we see that the mechanical sediments (shaly matter), of the 

 northern and north eastern margin of the old inland sea came 

 principally from the Canadian highlands. The Hudson River 

 group formed the shore line of most places, from the beginning 

 of the Medina epoch, both in New York and Canada as well as 

 along the " Cincinnati Arch." The eastern portion of the Pro- 

 vince of Ontario was covered by the limestones of the Trenton 

 group ; the central portion, b^' the great accumulation of dark 

 Utica shales^ and these last by shales with intercalated limestones 

 and sandstones of the Hudson epoch, extending nlong their western 

 margin, and forming the north-eastern shores of the sea, as de- 

 veloped at the beginning of the Silurian Age (proper), in the 

 region from what is now the western end of Lake Ontario to 

 Oeorgian Bay. 



It may be noticed that the limit of the Utica shales is not 

 west of the meridian of the Niagara River. At the close of 

 the Cambro-Silurian Age the deposits belonging to that period 

 extended much farther southward than at presejit, probably to a 

 latitude not far north of the southern shores of Lake Ontario 

 — at least, in its eastern extension, [t was in this soft material 

 that the lake basin was subsequently excavated, the erosion having 

 extended but a few miles into the Niagara limestones, and their 

 underlying shales, and left the escarpment in bold relief. 



Now, on examining the sediments south of the Canadian shores 

 of those days, we find only thin beds of shale in the more eastern 

 deposits, but these gradually thicken in extending westward, 

 until, in the neighbourhood of Rochester, they amount to 80 

 feet (the place being south of the shores composed of Utica shale) ^ 

 Again, the shales begin to thin out at Thorold, Ontario, where 

 they amount to fifty feet, while thirty miles westward, as at 

 Dundas, they are only a few feet thick, and almost entirely dis- 

 appear after turning the ancient Cape and passing west of the 

 line from this town to Lake Huron, as the waters, there, were 

 protected from the muddy eastern currents. The northern end 

 of the sea was not subjected to the influx of mud to any extent, 

 us in that direction the shores were adjacent to the old crystal- 

 line tlurouian and other mountains. However, more shales 

 make their appearance in the western area, having been derived 



