THE LOWER AND UPPER SILURIAN AGES. 57 



Mountains and the Alleghanies. Between tliese the 

 extensive triangular area now constituting the greater 

 part of North America, was a shallow oceanic plateau, 

 sheltered from the cold polar currents by the Lauren- 

 tian land on the north, and separated by the ridges 

 already mentioned from the Atlantic and Pacific. It 

 was on this great plateau of warm and sheltered ocean 

 that what we call the Silurian fauna lived; while of 

 the creatures that inhabited the depths of the great 

 bounding oceans, whose abysses must have been far 

 deeper and at a much lower temperature, we know 

 little. During the long Silurian periods, it is true, 

 the great American plateau underwent many revolu- 

 tions ; sometimes being more deeply submerged, and 

 having clear water tenanted by vast numbers of corals 

 and shell-fishes, at others rising so as to become 

 shallow and to receive deposits of sand and mud ; but 

 it was always distinct from the oceanic area without. 

 In Europe, in like manner, there seems to have been a 

 great internal plateau bounded by the embryo hiUs of 

 Western Europe on the west, and harbouring a very 

 similar assemblage of creatures to those existing in 

 America. 



Further, during these long periods there were 

 great changes, from a fauna of somewhat primordial 

 type up to a new order of things in the Upper Silu- 

 rian, tending toward the novelties which were in- 

 troduced in the succeeding Devonian and Carboni- 

 ferous. We may, in the first place, sketch, these 

 changes as they occurred on the two great continental 



