492 



CENOZOIC TIME. 



These fresh-water or lake deposits are, as stated, of all periods from 

 the Eocene to the Pliocene. The Eocene existed over the Summit 

 region, between the Wahsatch and the Front Range of the Rocky 

 Mountains — one lake-basin north of the Uinta Range, in the Green 

 River Basin, and another south, the latter reaching to New Mexico. 

 The Miocene covered part of the eastern slopes along the White River, 

 between the parallels of 40° and 44° — the White River Lake-basin of 

 Hayden ; also another in Central Oregon, along John Day River. A 

 great Pliocene lake spread over the Miocene region of the Eastern 

 slopes, and south nearly to the Mexican Gulf, and is called the Nio- 

 brara Lake-basin by Marsh ; there was also another in Nevada ; and 

 a third in the North Park. The Eocene beds in the Mountains have 

 a thickness of about 10,000 feet. In the Fort Bridger region, the strata, 

 owing to erosion by rills and streams from the rains, stand in isolated 

 earthworks or embankments, pyramids and spires, over the great 

 plain, looking like a field of desolate ruins. Such areas in the Western 

 Tertiary are called Mauvaises Terres, or Bad Lands, this name having 

 been originally applied to one of the kind in the White River region. 



Over the Coast region of California, the Tertiary formation is of 

 marine origin, and has a thickness of at least 3,000 or 4,000 feet. 



The Tertiary strata often vary greatly in character, from mile to 

 mile. Instead of great strata of almost continental extent and uni- 

 formity, as in the Silurian, there is the diversity which exists among 

 the modern formations of a sea-coast. • 



Off our present coasts, we find in one spot mud beds, with oysters or 

 other Moll usks ; in another region, great estuary fiats ; a little higher, 

 on the same coast perhaps, accumulations of beach sands with worn 

 shells, changing in character every few rods. The changes in the 

 Tertiary strata are often equally abrupt. It should be noted also that 

 coral limestones are now in progress off the Florida coast ; and, on 

 other shores, coarse shell-limestones. Still further, to comprehend the 

 diversity in the deposits, it is necessary to remember that, by the 

 throwing up or removal of embankments on coasts, or by change of 

 level, salt-water marshes or estuaries become brackish-water, or wholly 

 fresh-water, and the • reverse, — each change being attended with -a 

 change in the living species of the waters. 



The rocks are of the following kinds : beds of sand or clay, so soft 

 as to be easily turned up by a shovel ; compact sandstones, useful for a 

 building-stone, though not very hard ; shell-beds, of loose shells and 

 earth, the shells sometimes unbroken, in other cases water-worn ; shell- 

 rocks and calcareous sandstones, consisting of pulverized shells and 

 corals, firmly cemented and good for building-stone, as at St. Augus- 

 tine ; true marls, or clays containing carbonate of lime from pulverized 

 shells, and hence effervescing with the strong acids ; compact solid 



