144 ORIGIN OF THE EAKTH. 



necessarily have accompanied the great Chalk formation, and 

 closed immediately prior to the commencement of the present 

 or Alluvial period. It was distinguished from the previous 

 stage of terrestrial developments, mainly by its lacustrine, 

 volcanic, and fluvatile conditions, and by the erosive, leveling, 

 and harmonizing operations which, especially near the close of 

 the period, occurred on the earth's surface. These conditions 

 were evidently an improvement upon previous ones. The 

 earth became more extensively diversified by mountains and 

 valleys, forests, fields, and running streams. The quantity of 

 upland and fertile soil was greatly increased ; the atmosphere 

 was freed from previous pestilential vapors ; the climates were 

 rendered more salubrious, and all things were more compat- 

 ible with the existence of higher species in the organic king- 

 doms. Accordingly, even in the lower strata of this formation, 

 there are found the remains of animals of decidedly mam- 

 miferous species. These are of the order Pachydermata 

 (thick-skinned), and of comparatively low organization. But as 

 conditions advanced and new strata were deposited, higher 

 species successively made their appearance, organic life all the 

 while assuming more analogy to existing types, until, toward 

 the close of the period, there was, in many instances, an actual 

 shading off into species which now inhabit the earth. This 

 latter remark is equally applicable to the vegetable, as it is to 

 the animal, kingdom. 



About the close of this period, there appears to have been 

 a remarkable fall of atmospheric temperature, accompanied 

 by a submergence of the greater portion of land in the 

 northern and temperate regions, in seas filled with floating 

 icebergs. These icebergs, frequently reaching to the bottom 

 of the ocean, have scraped along over the earth's surface, 

 clashed violently against its prominences, torn fragments of 



