60 THE STOEY OF THE EARTH AND MAN. 



stone, constituting the Caradoc beds of Britain and 

 the Utica and Hudson River groups of America. 

 During the deposition of these, the abounding life of 

 the Siluro-Cambrian plateaus died away, and a middle 

 group of sandstones and shales, the Oneida and Medina 

 of America and the Mayhill of England, form the base 

 of the Upper Silurian. 



But what was taking place meanwhile in the oceanic 

 areas separating our plateaus ? These were identical 

 with the basins of the Atlantic and Pacific, which 

 already existed in this period as depressions of the 

 earth's crust, perhaps not so deep as at present. As 

 to the deposits in their deeper portions we know 

 nothing ; but on the margin of the Atlantic area are 

 some rocks which give us at least a little information. 



In the earlier part of the Silurian period the enor- 

 mous thickness of the Quebec group of North America 

 appears to represent a broad stripe of deep water 

 parallel to the eastern edge of the American plateau, 

 and in which an immense thickness of beds of sand 

 and mud was deposited with very few fossils, except 

 in particular beds, and these of a more primordial 

 aspect than those of the plateau itself. These rocks 

 no doubt represent the margin of a deep Atlantic 

 area, over which cold currents destructive of life were 

 constantly passing, and in which great quantities of 

 sand and mud, swept from the icy regions of the 

 North, were continually being laid. The researches 

 of Dr. Carpenter and Dr. Wyville Thomson show us 

 that there are at present cold areas in the deeper 



