Testimony of Geology. 67 



for the quantity of water in the Colorado is less, the rock is much harder 

 and the fall per mile is much less. Moreover, Powell shows that the 

 river runs across the anticlinal ridges and synclinal valleys, which proves 

 that the elevation of these ridges was so slow that the river had time to 

 cut its way across them faster than they rose. Otherwise it would have 

 been diverted from its course and compelled to traverse one of the syn- 

 clinal valleys. This cutting has been very slow and the work began a 

 ver}^ long time ago. And yet it certainly began since the Jurassic times, 

 and probably since the Cretaceous. Since the estimates of the table are 

 only one-ninth to one-sixth as much as this canyon gives, they may be 

 considered moderate. The proofs of age drawn from other gorges, 

 and from continental erosions are equally conclusive. 



The spaces between man}' of the formations indicate pause in rock 

 building, ordinarily accompanied with oscillations or changes of level, 

 often of the most radical character and consuming vast periods of time. 

 No correct idea of geological history or of the history of evolution can 

 be got without some understanding of these pauses. Their duration is 

 largety conjectural, but can sometimes be reduced within probable lim- 

 its. Where one series of rocks rests upon another in unconformity, it 

 proves that the lower series has been tilted up, more or less eroded and 

 then sunk below the sea to receive the top series, because when first laid 

 down, all sedimentary rocks are practically level. If the number of 

 feet of erosion can in any case be estimated, the table multiplies that 

 number by 5,000, as the number of years required to erode one foot, 

 using the Mississippi again as the average, and taking no account of the 

 time occupied in bringing the lower series to the top of the water after 

 its first accumulations cease, or sinking it below the water before its 

 second accumulations commence. 



These tables make the time since the beginning of the Laurentian, 

 289,500,000 years. The estimate, I have no doubt, is many times too 

 conservative. That of Volger is 648,000,000. 



Time is long: and enough lias elapsed for all the requirements of the 

 construction of plants and animals from the mineral materials. 



