THE AGE OF THE HABITABLE EARTH 285 



pre-Cambrian epoch, during which organic evolution must have 

 been taking place, though only somewhat doubtful indications 

 of organic remains have been met with. Probably the pre- 

 Cambrian rocks originally contained abundant fossils, which have 

 been destroyed by the treatment to which they have been sub- 

 jected. All the subsequent periods of the earth's history, 

 however, are represented by rocks containing organic remains in 

 greater or less abundance. These deposits indicate a division of 

 the earth's history since pre-Cambrian times into three main 

 eras primary or palaeozoic, secondary or mesozoic and tertiary 

 or cainozoic. Each of these three is subdivided into a series of 

 epochs, as shown in the accompanying table, in which are also 

 mentioned some of the principal rock formations by which the 

 different epochs are represented in the British Islands. 



The thickness of the strata deposited during each period differs 

 greatly in different parts of the world, and in North America the 

 various formations appear to be much better developed than in 

 North Western Europe. It is of the utmost importance that our 

 estimates of this thickness should be as accurate as possible, for 

 they constitute important data from which to calculate the age 

 of the habitable earth and the duration of the different geological 

 periods. 



Sir Archibald Geikie, 1 in 1892, very cautiously estimated the 

 total thickness of the stratified rocks, where most fully developed, 

 from the bottom of the Cambrian onwards, as not less than 

 100,000 feet. He also estimated that it would take from 730 to 

 6,800 years to add a foot to this thickness by accumulation of 

 material derived from the denudation of the land. Of course the 

 actual time must vary very greatly according to the nature of the 

 material deposited and the agencies at work. There is, however, 

 no reason to suppose that denudation and sedimentation took place, 

 on an average, more rapidly in past geological time than at the 

 present day. If, then, we take the average between the most 

 rapid and the slowest rate of denudation and sedimentation, and 

 the thickness of the stratified rocks as calculated by Professor 

 Geikie, we arrive at 376,500,000 years as the age of the habitable 

 earth exclusive of the pre-Cambrian period. 



Sir Archibald Geikie appears, however, to have assumed for 

 the purposes of his calculation that material derived from the 

 wearing away of the land has been spread out over an equal area 

 1 Presidential Address, British Association, Edinburgh, 1892. 



