THEIR ECONOMIC PRODUCTS. 211 



river-side ; and huge amphibia (halitheres, diuotheres, and 

 crocodiles) lazily sunning themselves on the islets and mud- 

 banks of the far-spreading delta ! Plain, lake, estuary, and 

 sea, teeming alike with life during the warmer eocene; 

 abounding in milder forms during the still genial miocene ; 

 and gradually assuming more temperate aspects as the 

 pliocene approached more nearly to the ordainings of the 

 current epoch. Such were the leading aspects of the Ter- 

 tiary Times times enjoying the genial surroundings of a 

 peculiar geographical distribution of sea and land, and only 

 brought to a close by new terraqueous arrangements inimi- 

 cal (over the greater portion of the northern hemisphere at 

 least) to vegetable and animal luxuriance. 



As Economic Repositories, the tertiary formations, though 

 greatly inferior to some of the older systems, are not with- 

 out their local importance. Brick and pottery clays, sands 

 for glass-making, fuller's earth, tripoli or polishing-stone,* 

 gypsum or plaster -of -Paris stone, limestones, burrh for 

 millstones, and a great variety of lignites or brown-coals 

 with their associated gums and amber, t are among their 



* These polishing, infusorial, or microphytal earths are among the 

 most wonderful of tertiary and recent accumulations, alike from their 

 origin and the vast thickness to which they sometimes attain. Accord- 

 ing to Professor Dana, the infusorial earth of Virginia is in some places 

 30 feet thick, extends from Herring Bay in the Chesapeake, Maryland, to 

 Petersburg, Virginia, or beyond, and is throughout an accumulation of 

 the siliceous remains of microscopic organisms, mostly Diatoms. A still 

 thicker bed, exceeding 50 feet, exists, according to Mr W. P. Blake, on 

 the Pacific at Monterey, and is white and porous like chalk. The 

 " polishing powder " of Tripoli and Bilin, and the " mountain meal" of 

 Tuscany, Sweden, and other countries, are kindred accumulations. 



t In some of the later lignites, like those of Northern Germany and 

 Bunnah, amber and other fossil gums and resins are found in all but 

 their original attachment to the trees from which they were exuded. 

 And in these ambers the insects of the tertiary forest are often as per- 

 fectly preserved as the specimens in the cabinet of the most fastidious 

 collector. 



