532 CENOZOIC TIME — MAMMALIAN AGE. 



an elevation through some part of this altitude. This is shown, as 

 Hayclen observes, in the displacements which the Eocene beds 

 have undergone, especially in the vicinity of the mountain-ridges 

 about the summits of the mountains, and in which the Miocene 

 did not to an equal extent participate. 



West of the Black Hills, according to this author, the dip is 5 to 10 degrees. 

 Along the Big Horn Mountains there is a similar dip, and in some places even 

 greater. Between the western extremity of these mountains and the North 

 Platte the beds are nearly vertical, so that the harder sandstone layers stand in 

 projecting lines over the surface, the softer intermediate clayey layers being 

 worn away. Along the Laramie range, and the Wind River Mountains, also, 

 the strata are more or less inclined. The Wind Biver beds, which are supposed 

 to succeed to Lignite formation (p. 510), also partook in the dip, though to 

 a less amount ; and this is part of the evidence in favor of their being earlier in 

 age than the White Biver or Titanotherian beds. The latter are but slightly 

 disturbed, if at all. 



The White Eiver (Miocene) beds occur, however, high up on the 

 mountain-slopes, showing that they have participated in a more 

 recent elevation. And it is probable that some 1000 or 2000 feet, 

 at least, of the altitude were added to the height of the summit 

 after the Miocene era. 



On the Pacific coast in Oregon and California there is evidence, in 

 the present height of the Miocene beds, of an elevation of 1500 to 

 3000 feet after the Miocene era. 



The alternation, in the Eocene of Mississippi, of Lignite beds with 

 those containing marine shells indicates an alternation of condi- 

 tions in the course of the progress of the formations which were due 

 either to oscillations of level in the waters, or to the damming up 

 and breaking away of coast-barriers : through such catastrophes 

 the marine species probably met with that more or less complete 

 extermination which intervened between the Claiborne, Jackson, 

 and Vicksburg epochs. Sharks, having the free ocean to escape 

 in, underwent less change of species through the Tertiary period 

 than animals of other kinds. The remains of the Zeuglodon are so 

 widely distributed over the Gulf-border Tertiary, and so abundant, 

 that nothing less than a sudden elevation of the land in the Jack- 

 son epoch, or the rush inland of a great earthquake-wave, could 

 have produced the effects observed. 



The North American Continent, which since early time had been 

 gradually expanding in each direction from the northern Azoic, 

 eastward, westward, and southward, and which, after the Palaeozoic, 

 was finished in its rocky foundation excepting on the Atlantic and 

 Gulf borders and the Western Interior region, had reached at the close 



