PALAEONTOLOGY AND GEOLOGY 



lution these unreliable and misleading chronological 

 tables reckoned in years. 



In spite of the necessity of postulating that we 

 must estimate the time involved in the formation of 

 strata by applying conditions approximately those of 

 today, we have unmistakable evidence that the forces 

 of nature vary greatly at different times. The deposi- 

 tion of sedimentary rocks depends on the rate of rise 

 or fall of the floor of the sea, on the amount of rain- 

 fall, temperature, the composition of the air, the 

 abundance of animal and vegetable life, and many 

 other factors. In addition, during periods of no depo- 

 sition in an area, there is no time record and, as all 

 deposition in one place means erosion somewhere 

 else, we can assume as a general law that every foot 

 of rock which we find today is the unknown remnant 

 of more than a foot of deposition or, translated into 

 time, every geological time interval must be length- 

 ened by an unknown amount. Finally, equal thick- 

 nesses of strata do not signify equal intervals of time, 

 as the rate of deposition is dependent closely on the 

 rate of subsidence of the floor on which the deposit is 

 laid. The eminent geologist, Sir Archibald Geikie, is 

 fully alive to the danger of such estimates of past 

 time. He is of the opinion that: "The few centuries, 

 wherein man has been observing nature, obviously 

 form much too brief an interval by which to measure 

 the intensity of geological action in all past time. 

 . . . The present may be an era of quietude." 



I 149 1 



