492 CENOZOIC TIME. 



the Middle Eocene to the Pliocene : the Eocene occurring about Fort 

 Bridger ; the Miocene, in the Upper Missouri region, about White 

 River, in Colorado, etc. ; and the Pliocene, on the Loup Fork of the 

 Platte, the Niobrara, etc. The Fort Bridger region has been described 

 as an immense basin, the bed of an ancient lake, sterile and almost 

 treeless, having the Uintah Mountains on the south, and the far distant 

 Wind River Mountains on the North. The Tertiary beds, indurated 

 clays and sand, are 8,000 feet thick and nearly horizontal. The strata 

 have been eroded by rills and streams from the rains, and 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, where the beds are Miocene Tertiary. 



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 Mollusks ; in another region, great estuary flats ; 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, encroaching fresh waters 

 destroying the marine species, and so on. By considering carefully 

 all the various conditions incident to a coast from these sources, the 

 ever varying character of the Tertiary beds will be appreciated. 



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 



