GENERAL PRINCIPLES 9 



of the sea, deep or shallow, close to or far from land, etc. Litho- 

 logic character alone may give some idea as to the depth of water 

 and proximity to land where a given stratum was deposited, but 

 the presence of considerable numbers of terrestrial organisms gives 

 important additional data. Thick limestones filled with fossil 

 Corals point to long-continued conditions of clear sea water. Tree 

 stumps, on the other hand, with roots still in their original position, 

 plainly prove a former land surface. By means of fossils, many 

 land areas have been proved to have existed as effective barriers to 

 migrations of marine organisms. Certain lands now separated by 

 water may be shown to have been formerly connected, as was true 

 of Alaska and Siberia, by a land connection across Bering Strait. 

 Also the fossils found in the rocks of the Isthmus of Panama show 

 that North and South America were there connected at a compara- 

 tively recent time in earth history. 



3. Past climatic conditions. Some strata afford an idea of 

 the climatic conditions under which they were laid down. Thus 

 salt and gypsum beds, more or less associated with certain red 

 sandstones or shales, indicate an arid climate at the time of their 

 formation. But the study of fossils is much more fruitful in this 

 connection. Certain strata in southern England contain fossil 

 Palms, Gourds, Crocodiles, etc., thus proving a subtropical climate 

 for the time of their origin. Other strata, representing a later 

 date in southern England, carry remains of Arctic animals and 

 hence indicate a cold climate for that time. The finding of 

 Walrus remains in New Jersey and Musk-ox remains in Arkansas 

 indicate a former colder climate for those regions. Again, many 

 fossil Palms, Ferns, and other temperate to subtropical plants, as 

 well as animals, clearly point to former warmer climate in those 

 same regions. 



Much strong evidence for climatic conditions over various por- 

 tions of the earth during different geologic periods has been fur- 

 nished by the study of true marine organisms. Certain kinds of 

 Corals live only in shallow tropical seas, and so, if in any region 

 we find a bed of limestone rich in Corals of this kind, it is to be in- 

 ferred that this limestone was formed in warm, shallow sea water. 

 Such coral limestones are known even in the interior of North 

 America. 



In deducing climatic inferences, as above explained, certain 

 care must be exercised, because we are not justified in assuming that 



