GEOLOGICAL CHRONOLOGY. 



5 



of them in the older systems, that they must have re- 

 quired vastly greater time. Taking these criteria into 

 account, it has been estimated that the time-ratios for 

 the first three great ages may be as one for the Kainozoic 

 to three for the Mesozoic and twelve for the Palaeozoic, 

 with as much for the Eozoic as for the Palaeozoic. This is 

 Dana's estimate. Another, by Hull and Houghton, gives 

 the following ratios : Azoic, 34*3 per cent. ; Palaeozoic, 

 42*5 per cent. ; Mesozoic and Kainozoic, 23*2 per cent. 

 It is further held that the modern period is much shorter 

 than the other periods of the Kainozoic, so that our 

 geological table may have to be measured by millions of 

 years instead of thousands. 



We cannot, however, attach any certain and definite 

 value in years to geological time, but must content our- 

 selves with the general statement that it has been vastly 

 long in comparison to that covered by human history. 



Bearing in mind this great duration of geological time, 

 and the fact that it probably extends from a period when 

 the earth was intensely heated, its crust thin, and its con- 

 tinents as yet unformed, it will be evident that the con- 

 ditions of life in the earlier geologic periods may have- 

 been very different from those which obtained later. 

 When we further take into account the vicissitudes of 

 land and water which have occurred, we shall see that 

 such changes must have produced very great differences 

 of climate. The warm equatorial waters have in all 

 periods, as superficial oceanic currents, been main agents 

 in the diffusion of heat over the surface of the earth, and 

 their distribution to north and south must have been 

 determined mainly by the extent and direction of land, 

 though it may also have been modified by the changes in 

 the astronomical relations and period of the earth, and 

 the form of its orbit.* We know by the evidence of 



* Croll, " Climate and Time/ 



