56 GEOLOGICAL BIOLOGY. 



32,000 to 35,000 feet. There are now canons from 5000 to 

 6000 feet deep, excavated entirely since the Eocene period. 



(3) It is believed that all the lava outflows in the North- 

 west, which cover 150,000 square miles along the Columbia 

 River and the neighboring states, and through which the 

 Columbia has cut a channel, in some cases, from 3000 to 

 4000 feet deep, were erupted and laid down since Miocene 

 Tertiary time. 



(4) Niagara River gorge, from the falls down to the whirl- 

 pool, and thence to the cliffs of the lake at Lewiston, it is 

 estimated, was cut out since the retreat of the glacial ice from 

 the surface of the northern part of the continent, and this is 

 believed by many geologists to represent closely the length of 

 time since man first appeared upon the earth. The gorge is 

 7 miles long, one fourth of a mile wide below, narrower 

 above the whirlpool, and varies from 200 to 500 feet in depth.* 

 The length of time required for its excavation is estimated to 

 have been from 10,000 to 32,000 years. Taking Dana's gen- 

 eral estimate of relative length of time, it is seen that the 

 time since the Cretaceous is not over one sixteenth of the time 

 from the beginning of the Cambrian, and that the length of 

 Quaternary time is not over one third that of the Tertiary. 

 Whatever be the actual length of time taken for these and 

 similar geological processes, it is evident that the same forces 

 working at the same rate would require but the extension of 

 time to include the whole history of the earth. 



Data upon which Time-estimates are Made. Although we 

 cannot go into full particulars respecting the theories proposed 

 to determine the time-limits and extent of the geological ages, 

 a few of the prominent attempts may be cited. The principal 

 data upon which the theories have been based are as follows : 



(i) Physical and Astronomical. Estimates from the earth's 

 heat, its rate of cooling, and the radiation of heat into space. 

 (Kelvin.) 



Estimates from influence of tidal friction, and thence to 

 the length of time since the moon was separated off from the 

 earth. (Darwin, G. H.) 



*See J W. Spencer, "The Duration of Niagara Falls:" Am. Jour. Sci., 

 vol. XLVIII. p. 455. December, 1894, 



