CENOZOIC TIME — TERTIARY. 887 



universal, for in some regions the sands from shells and corals were made 

 into hard limestones, as they are now, and over areas of great extent. 

 Moreover, firm shales and sandstones occur that are like those of early time. 

 Besides, there are thick beds of greensand, like those of the Cretaceous 

 formation in constitution, and equally valuable as a fertilizer. There are 

 also beds of coal or lignite associated with some of the deposits. 



Beds of siliceous organisms, Diatoms, Radiolarians, and Sponge spicules, 

 have sometimes much thickness, and are occasionally partly consolidated 

 into opal. 



The rocks of the lacustrine and terrestrial deposits are generally fine- 

 grained, and either feebly indurated sandstones, soft straticulate clays 

 passing into shales, or soft fragile limestones of fine grain; but these 

 soft kinds graduate into harder and sometimes into coarser varieties. 

 They have derived their great thickness in the usual way ; that is, through 

 a gradual subsidence attending the deposition from waters of the region. 

 On the coast of Florida, some beds have been converted partially into 

 phosphates (or phosphatized), by water filtrating through overlying guano 

 deposits. In the Eocky Mountain region and over the Pacific slope occur 

 deposits, sometimes hundreds or thousands of feet thick, made of volcanic 

 ashes. There are also coarse volcanic conglomerates or breccia. The 

 volcanic beds sometimes cover the stumps of many successive growths 

 of forests (page 135) ; and the finer kinds occasionally contain remains of 

 the Beetles, Butterflies, and other Insects of the period. 



Lignite beds also occur locally over the country. One of the most noted 

 of them is that of Brandon, Vt., which is probably of Eocene origin. It 

 is associated with a bed of limonite. 



Denudation was universal over the exposed continental surface, as in 

 all past time, dissecting and degrading mountains, and making fluvial 

 deposits as well as lacustrine. The Auriferous gravels of the western 

 slope of the Sierra Nevada are largely fluvial deposits of Tertiary origin, 

 as shown by J. D. Whitney in his Geological Re^iort on Galifornia (1865), 

 and much more fully in his Auriferous Gravels of the Sierra Nevada 

 (1880). The plants found in the gravel beds indicate, according to Les- 

 quereux, a Miocene and Pliocene age; but Whitney regards the formation 

 as representing the whole of the Tertiary. It probably began in the Cre- 

 taceous period. As Le Conte states, the detritus of the old gravels is in 

 general exceptionally coarse, showing strong currents. 



1. Sea-border Areas. 



I. Eocene. — Along the Atlantic and Gulf borders (see map, page 881), 

 the Tertiary belt is very narrow and interrupted through New Jersey ; it is 

 broader in Maryland and Virginia, and still broader in South Carolina. But 

 the formation is best displayed on the Gulf border. The inner limit, or that 

 against the Cretaceous in the Carolinas and the Gulf region, is over 100 miles 



