342 GEOLOGY. 



European continent, on the other hand, orogenic disturbances do 

 not appear to have taken place at the close of the Ordovician. In 

 Europe, as in America, the great disturbances took place where thick 

 bodies of sediment had been accumulated, or else the beds were 

 greatly thickened by the disturbances. 



The Ordovician system of Europe, like that of America, is divided 

 into several groups of strata, but it is impossible to correlate these 

 subdivisions with those of North America with anything like accu- 

 racy. In Great Britain the four principal divisions, beginning with 

 the oldest, are (1) Tremadoc, (2) Arenig, (3) Llandeilo, and (4) Cara- 

 doc or Bala. 



In other continents the Ordovician strata have usually not been 

 separated from the overlying Silurian, but they are known to have a 

 wide distribution in the Arctic regions (Kennedy Straits, Boothia, 

 Cornwallis, Griffith, North Devon, East coast of Greenland, etc.). In 

 south-central Siberia they are said to be represented by red beds, with 

 salt and gypsum. 1 In northern China, the Ordovician system is repre- 

 sented by 3000 to 4000 feet of limestone, which overlies the Upper 

 Cambrian conformably. 



Climate. 



Neither in Europe nor in America is there decisive evidence that 

 climatic zones were distinctly marked. All that is known of the life 

 of this era would seem to indicate that the climate was much more 

 uniform than now throughout the areas where the strata of the period 

 are known. The fact that the Ordovician rocks have been identified 

 in the far north (in North Devon, the west coast of King William's 

 Land, Boothia, etc.) by their fossils, indicates that the climatic con- 

 ditions of North America and Europe must have been less diversified 

 than now. 



ORDOVICIAN LIFE. 



Close sequence upon the Cambrian. — To the fact that no great 

 physical change took place in the passage from the Cambrian to the 

 Ordovician deposits, as usually defined, 3 is now to be added the cor- 



1 De Lapparent, op. cit., p. 807. 



2 Blackwelder, Carnegie Institution. Unpublished evidence. 



3 Were the dividing plane placed at the unconformity above the Calciferous (Beek- 

 mantown of the east, Lower Magnesian of the Upper Mississippi Valley), an appre- 

 ciable change in the life would have to be recognized, but it would not be a radical 

 one. For North America, the plane of the unconformity appears to be the better 

 dividing horizon. 



