376 ORDOVICIAN PERIOD 



24,000 feet maximum). 1 Locally (Wales), nearly half the system is 

 of igneous rock, including sheets of lava and beds of pyroclastic 

 material. This is one of the most extensive, as well as one of the 

 most ancient, volcanic tracts of Europe. From north England and 

 Wales the system thins in all directions. 



The Ordovician of Europe is generally conformable on the 

 Cambrian, but over considerable areas it is unconformable beneath 

 the Silurian. 



In other continents the Ordovician strata have not, as a rule, 

 been separated from the overlying Silurian, but they are known 

 in South America, Australia and China. 



Duration and Climate 



The duration of the Ordovician is perhaps no better known than 

 that of the Cambrian, but the period was probably somewhat 

 shorter than its predecessor. 



Neither in Europe nor in America is there decisive evidence of 

 distinct climatic zones. All that is known of the life would seem to 

 indicate that the climate was much more uniform than now where 

 the strata of the period are known. The fact that Ordovician 

 rocks have been identified in the far north by fossils akin to those of 

 low latitudes, indicates that the climatic conditions of North America 

 and Europe must have been less diversified than now. This appar- 

 ent lack of diversity of temperature through wide ranges of latitude 

 is one of the unexplained problems of geology. Its solution is 

 possibly to be found in a much higher average temperature of the 

 ocean. 2 If the body of the ocean- water was relatively warm (in- 

 stead of cold as now), it would have done much to counteract the 

 effect of slight insolation in high latitudes during the cooler part of 

 the year. 



LIFE 



From Cambrian to Ordovician, there was no pronounced break 

 in the succession of life. The time from the beginning of the first 

 to the close of the second of these periods appears to have been 

 one long eon of progressive development and expansion of life. 

 The fossil record of the Ordovician is fuller than that of the Cam- 

 brian. This is due partly to an increase in fossilizable forms, partly 



1 This measurement is doubtless subject to the strictures set forth on p. 355. 



2 Chamberlin and Salisbury, Earth History, Vol. Ill, pp. 437-445. 



