THE CRUST OF THE EARTH. 



of a frog are twice as large as one of these animalcules. As the 

 Polirschiefer of Bilin is slaty, but without cavities, these animal- 

 cules lie closely compressed. In round numbers, about twenty- 

 four millions would make up a cubic line, and would, in fact, 

 be contained in it. There are 1728 cubic lines in a cubic inch ; 

 and therefore a cubic inch would contain, on an average, about 

 forty-one thousand milHbns of these animals. On weighing a 

 cubic inch of this mass, I found it to be about 220 grains. Of 

 the forty-one thousand millions of animals, a hundred and 

 eighty-seven millions go to a grain ; or the siliceous shield of 

 each animalcule weighs about ^ millionth part of a grain." 



The remains of these Infusoria are often found in abundance in 

 flints, opals, and more especially in the earthy matter which 

 envelopes the translucent parts. They exist in large quantities, 

 also in most marls, especially in those of lacustrine depositions 

 in calcareous slates of the same formation, and in all chalk strata. 

 They form the chief part of the deposits which fill the gulfs and 

 arms of the ocean, and are found in all the earthy deposits raised 

 from the bottom of the waters in ancient and modern times. 

 They exist in strata sixty feet thick in the low plains of "Western 

 Germany, at a depth greater or less under the sands of those 

 countries. It is a remarkable fact, that one of the strata on 

 which the city of Berlin is placed, is formed of the shells of 

 Infusoria which still live and are propagated and sustained, 

 doubtless, by the waters of the Spree, on which that city is built. 



M. Ehrenberg has described numerous fossil genera and species 

 of Infusoria found in all parts of the world, and in different 

 strata, a few of which are represented on a high magnified scale 

 in fig. 107. 



178. At whatever heights upon the land fresh- water shells and 

 the remains of land animals may be found in the sedimentary 

 strata, no surprise can be excited, since it is perfectly conceivable 

 that at various epochs any portions of the land, whatever be its 

 level, may have been submerged by lakes or overflown by rivers. 

 But we find, also, at all levels, no matter how high, even at 

 the summits of lofty ranges of mountains, marine deposits in 

 strata of immense extent and vast thickness. 



179. In many places an analysis- of the strata shows the most 

 curious alternations between fresh-water and marine deposits, 

 which can only be explained by the supposition that the crust of 

 the globe at these places has undergone a succession of elevations 

 and depressions, and that after being submerged for an indefinite 

 period, and receiving marine deposits, it was then upheaved so as 

 to become dry land ; and that during another indefinite period it 

 was peopled by land and fresh- water animals, and covered with a 



132 



