392 THE CHANGING GENERATIONS 



some places where rocks formed during the depositional gaps are still 

 accessible. These give us a fragmentary record of what went on in the 

 seas during the period of continental elevation. But at very long intervals 

 unusually great "revolutions" occurred, in which all the continents stood 

 so high and for so long a time that little or no record of the interval has 

 been found. During these times erosion also destroyed a vast amount of 

 the earlier records. These great breaks are taken as the dividing points 

 between the major eras of earth history. For reasons pointed out later, 

 they are also times when profound alterations occurred in the world of 

 life, many ancient types becoming extinct, and the evolution of new types 

 being accelerated. 



The concept of time in relation to earth history. The student of 

 science must become accustomed to thinking in terms of several quite 

 different scales of time. Recorded history is only a few thousand years 

 old, and in the historical sense an event of 6,000 years past is very ancient 

 indeed. To the student of human evolution, whose field of inquiry com- 

 prises the Pleistocene or glacial epoch, an event of 6,000 years ago is 

 quite recent, whereas really ancient events occurred 250,000 or 500,000 

 years ago. To the geologist years are of little moment; he deals with a 

 vast period of time in which 6,000 years ago is the same as today and the 

 Pleistocene in its entirety is an insignificant part of the whole. If years 

 are insisted upon, he points out that the oldest known rocks, on the basis 

 of recent physical estimates, are perhaps 1 billion 900 million years old, 

 and that, since these do not record the beginning of earth history, a 

 figure of 2 billion 500 million years may be taken as a working hypothesis 

 for the age of the earth. l But the geologist and the biologist are much more 

 concerned with the relative age and relative duration of events, and here 

 they can speak with much greater assurance. Finally, to the astronomer, 

 the whole period covered by the history of the earth is an insignificant 

 fraction of the life of a single star such as the sun. 



The Geological Time Scale. By fitting together the fragments of the 

 rock record preserved in different regions, it has been possible to construct 

 a detailed time scale, in which names are given to the periods of earth 



1 This is not a mere guess, but the result of prolonged and careful investigation of 

 the problem from many angles. Data have been obtained based on the total thickness 

 of sediments and the probable rate of their accumulation ; on volume of rock removed 

 in relation to rates of erosion ; on amount of salt in the oceans relative to the amount 

 added annually by present-day streams, etc. All such methods of estimate have been 

 largely superseded by those based on the disintegration of uranium and thorium into 

 lead, which goes on at a constant and known rate. Determination of the extent to 

 which this disintegration has progressed in a sample of crystalline rock permits estima- 

 tion of the time of formation of the rock. To this and related methods we owe our pre- 

 sent concept of the magnitude of geological time. A discussion of the methods used in 

 dating Pleistocene and postglacial fossils and events will be found in Chap. XXX. 



