3 82 HISTORICAL GEOLOGY 



of years after it has become extinct in others. An example is found 

 in Australia to-day, where the indigenous fauna belongs to the early 

 Tertiary, 



Use of Fossils in Determining Physical Conditions. — A study of 

 the inclosed fossils usually tells definitely whether the rocks were 

 laid down in the sea, in a lake, or on land. Fossils also give a clue 

 to the depth of the water and the proximity of the shore. Corals 

 show that the deposits containing them were laid down in warm seas 

 some distance from land, or that the land was so low that little sedi- 

 ment was carried to the sea. Leaves and stems of plants as well as 

 the fossils of land animals indicate nearness to shore. 



The climate of the past is also told with considerable certainty 

 by fossils. For example, relatively recent travertine deposits of 

 northern France contain the canary laurel, a plant which blooms in 

 winter and which now grows in the moist climate of the Canary Is- 

 lands, where the temperature seldom falls below 59 F. It is evi- 

 dent, therefore, that when the canary laurel grew in northern France 

 the climate of that region was probably warm and moist. The occur- 

 rence in the Pleistocene deposits of Denmark and England of Arctic 

 willows which now grow only within the Arctic Circle is evidence of 

 a cool climate in the past in those countries. 



A typical example of the knowledge to be gained of the physical 

 geography and climate of a region by a study of the fossils is illus- 

 trated by the limestones of Wisconsin. These strata are composed 

 of practically pure limestone, being free from land sediments, and 

 contain fossil corals, crinoids, brachiopods, and the remains of other 

 marine animals. It is evident, therefore, that when the limestone 

 was accumulating, a sea spread over a portion at least of Wisconsin, 

 that the region in which the lime ooze was deposited was probably 

 far from land, and that the climate, as shown by the corals, was prob- 

 ably warm. 



Difficulties in Correlating Strata. — (1) When rocks have been over- 

 turned or faulted, older beds are sometimes found to rest on younger 

 ones. (2) In some regions a once widespread stratum may be repre- 

 sented now only by isolated patches which may be separated by dis- 

 tances of several miles. (3) Strata are sometimes separated by an 

 unconformity (p. 270) which may represent a lost interval of many 

 years. (4) The lithological character of a stratum may vary greatly 

 even over short distances. In every case, however, fossils if present 

 mil usually give definite knowledge of the relative age of the rocks. 



