388 THE SILURIAN PERIOD 



ern lands was probably reopened by this disturbance, by way of 

 the upper Hudson and Lake Champlain, for near Montreal the 

 beds of this series rest directly upon Ordovician strata (Utica 

 shales). The channels reaching southward from the Gulf of St. 

 Lawrence, which were extended by the depression, also received 

 limestone accumulations, of which remnants still are found in 

 Vermont, Maine, and the maritime provinces of Canada. South- 

 ward the principal area of the Lower Helderberg extends through 

 Pennsylvania to Virginia, and it recurs in western Tennessee 

 and Missouri, but has not been distinguished in Wisconsin or 

 Illinois. 



Silurian strata are not common in the Rocky Mountain region, 

 and the persistence there of comparatively deep water from Or- 

 dovician times makes it very difficult to distinguish the series and 

 stages which are so well marked in the East, and which were due 

 to the oscillations of level and the changes in the character of 

 sedimentation which occurred in the latter region. 



Foreign. — The division into northern and southern areas which 

 we found in the Ordovician, was maintained in Silurian times, and 

 the southern sea was as peculiar in its animal life as it had been 

 before, the northern being the typical Silurian which is found in 

 the other continents. In the west of Ireland, Wales, northern 

 England, and Scotland, Silurian beds accompany and overlie the 

 Ordovician, but the much greater development of limestone points 

 to a deepening of the water in those seas. The Wenlock lime- 

 stone of Great Britain, which corresponds to the American Ni- 

 agara, is, like the latter, largely coralline. In Scandinavia also 

 there is a great development of Silurian limestones, which extend 

 far into Russia. In the latter country the sea had retreated much 

 from its extension in the Ordovician, except toward the southeast, 

 where it was carried into Bessarabia. Most of the Russian Silurian 

 was formed in an interior sea, connected with that of southern 

 Europe. In the southern European countries, which display the 

 Bohemian type of development, ox fades, the Silurian rocks .have 

 nearly the same general distribution as the Ordovician. The two 

 svstems are also associated in the Arctic islands, in China, north 



