



GEOLOGICAL AGE OF THE GRAVELS. 



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need only refer to the extensive area in northeastern North America, where 

 there is no representation of any geological epoch between Palaeozoic and 

 post-Tertiary. It is impossible, in such a case as this, to suppose that the 

 missing formations, or any considerable portion of them, have once existed 

 there, and been removed by erosion. It is manifestly in the highest degree 

 improbable that such a vast region could have been so thoroughly denuded 

 of any considerable mass of overlying material that no vestige of it should 

 remain behind. It seems, therefore, to be a legitimate inference that there 

 were conditions unfavorable to the development of organic life on land dur- 

 ing long periods of time. Such seems, at all events, to have been the case 

 during the early portion of Tertiary times, on the western slope of the Sierra 

 Nevada ; for we have there no representation of the Eocene at all, and only 

 a very imperfect one of the Miocene. 



Indeed, while we find in the detrital beds accompanying the gravel a very 

 considerable number of both plants and animal remains, there is far from 

 being enough material to enable us to divide up the formation into groups. 

 The reasons for the poverty of the collections from the gravel-mining region 

 have been already given. In spite of these deficiencies, however, valuable 

 inferences can be drawn with regard to the relations of the life of the gravel 

 period and the more important questions which bear on the geological his- 

 tory of the region answered. The testimony of the fauna and the flora is 

 essentially to the same effect. The general facies is decidedly later Tertiary, 

 or Pliocene, while there are also both Miocene and Recent types present. 



If there were no fossil remains in these gravels and the accompanying 

 clayey and sandy strata, it would not have been possible to demonstrate that 

 they did not belong to the present epoch. It might have been shown that 

 during and since their deposition a very long period had elapsed, by point- 

 ing out the vast amount of time required for the erosion which has taken 

 place since the end of the gravel period, not to speak of the thickness and 

 complexity of the various masses, the accumulation of which must have occu- 

 pied a still longer time. Still, in spite of this, it might have been said that 

 we know little of how rapidly the work either of accumulation or erosion 

 may have gone on in former times, especially in a region lifted up into high 

 mountain ranges, and the centre of volcanic manifestations. Having the evi- 

 dence of fossils, there can be no hesitation in allowing all the time necessary 

 for the inorganic changes which have taken place in the gravel region, since 

 we must have it in order to afford an opportunity for those successive devel- 



