66 Dynamic Theory. 



remains. These are named in the first period in which they occur. 

 The period of the extinction of some of them is also given. The great- 

 est thickness of the strata is given, and there is an attempt to estimate 

 the geological times in years. Dana estimates that the deposition of lime- 

 stones as the remains of calcareous shells requires at least five times as 

 long as the deposition of sedimentary rocks. On this basis the relative 

 times of the Tertiary Mesozoic and Paleozoic would be nearly in the ratio 

 of 1, 2, 4. The Mesozoic of Germany is composed as follows : Triassic, 

 1,000 feet limestone and 2,400 of sedimentary deposits, Jurassic, 

 1,000 feet limestone, 4,200 of sediments, Cretaceous, 1,200 feet lime- 

 stone and 1,200 feet sediments. Multiplying the limestone by five in each 

 case and adding the sediments to the products we get for the Triassic, 

 7,400, Jurassic, 9,200, Cretaceous, 7,200. These figures, therefore, rep- 

 resent the beds as if the}' were all sedimentary. From calculations based 

 on the amount of sediment carried off by the Mississippi river, it is esti- 

 mated that it denudes its whole basin at the rate of one foot in five 

 thousand years. If it be assumed that the sediments are deposited over 

 an area one-fourth as great as that from which they are taken, and that 

 the erosion of the Mississippi valley is a fair average, it follows that rocks 

 are built at the average rate of one foot in 1,250 years. Applying this 

 rate to the Mesozoic of Germany we get nine and one-fourth millions 

 years for the Triassic, eleven and one-half for the Jurassic and nine for 

 the Cretaceous, or as stated in even numbers in the table, nine, twelve 

 . and nine millions respectively. The other ages are figured from this ac- 

 cording to proportions above. The thickness of the Eozoic in Bohemia 

 is 90,000 feet. Its age is estimated by the same rule in the table. The 

 Canadian Eozoic is about 49,000 feet thick, but separated by a period of 

 elevation, erosion and depression which would probably make its age as 

 great as the European. The Quarternary time is assumed at half the 

 Tertiary by some geologists, and so it is in the table. The Quarternary 

 is an age of erosion, and such estimates as there are, have to be founded 

 on the phenomena of denudation and erosion mostly. The formation .of 

 the great canyons and gorges in the Pacific states and western territories, 

 and of the Mississippi and Niagara rivers, is all recent or Tertiary. It 

 is estimated by Ly ell that the Niagara gorge forms at the rate of one foot 

 per annum. Desor put it at three feet in a century. Dana suggests 

 one inch per annum which is doubtless nearer the truth than either of the 

 other estimates. At this rate the formation of the gorge has already 

 occupied 380,000 years. Now the grand canyon of the Colorado is on 

 an average thirteen times as deep and it is about forty- three times as long 

 as the Niagara gorge, and if the conditions of erosion were equal, the 

 Colorado canyon would be 212,420,000 years old if the Niagara is 

 380,000. But the conditions seem to have been in favor of the Niagara, 



