TIME SPENT (PALEONTOLOGY) 475 



in the past were gradual and time consuming, exactly as they are seen 

 to be before our very eyes today. 



Such repeated rainfalls drain down the slopes of the newly emerged 

 land, and, after countless contributions from lesser streams, combine 

 into rivers which cut slowly into the elevated accumulations, of sedi- 

 mentary rock and wear it away. Thus, in the course of long eons of 

 time, a river with its abrasive sediment scours out and fashions a 

 gorge. 



In the case of the Grand Canyon the rushing Colorado River, now 

 down a mile deep from the rim of the gorge, is still grinding away 

 unceasingly at its uncompleted task of recording spent time. What 

 a majestic open diary of the passage of time ! 



Measures of Time 



The biologist finds it not only convenient but indispensable to 

 establish some sort of foot-rule by means of which the continuous 

 and incomprehensible past may be divided into understandable por- 

 tions. To this end, the stratified or sedimentary rocks of the geologist 

 prove to be of the greatest use. Even so, only through much persist- 

 ent study by experts has anything like a satisfactory time-scale been 

 evolved. 



Sedimentary rocks, for example, sandstones, limestones, and shales, 

 do not envelop the entire earth in continuous layers in the way that 

 an onion is made up. They occur only in patches here and there, 

 where once was water in which they could be deposited from the 

 surrounding land masses. However, when the various patches of 

 sedimentary rocks the world over are examined and compared, it is 

 quite possible to piece them together, like a jig-saw puzzle, into a total 

 column of layers one above the other. 



For purposes of identification and description, the time consumed in 

 the formation of this column may be divided into eras and sub- 

 divided into periods. While the opinion of experts may differ with 

 respect to the limits and details of these arbitrary divisions of past 

 time, there is universal agreement as to their orderly sequence. 

 Such a time-scale of eras and periods is given on the following page. 



In this time-scale stratified rocks can be employed as a standard 

 of estimation for only approximately the last half of known time, i.e., 

 45 per cent. The rocks of the Proterozoic and Archeozoic eras that 

 characterize the older approximate half, i.e., 55 per cent of the time- 

 scale, are either of the original fire-fused sort which has never been 



