768 THE POPULAR SCIENCE MONTHLY. 



almost entirely by their porous shells that the immense chalk cliffs 

 and downs along the English Channel and elsewhere have resulted 

 from masses of their shells, and that they thus bear the same relation 

 to the Cretaceous or Chalk as do plants to the Carboniferous or Coal 

 age. The very dust of those chalky regions was once alive ! Of course, 

 many other kinds of organisms helped to some extent in the forma- 

 tion of cretaceous deposits, but the great bulk was xmdoubtedly of rhi- 

 zopod origin. Standing at Dieppe, France, beneath the immense chalk 

 cliffs of the English Channel, one can hardly realize that these beds of 

 solid chalk, hundreds of feet in thickness, are the produce of such 

 diminutive beings. But when we reflect that the chalk is five hun- 

 dred and thirty-five feet high at Beachy Head, and five hundred feet 

 at Wendover Hill, that it has been bored into five hundred and ten 

 feet at Diss, in Norfolk, and that its average thickness has been esti- 

 mated by reliable geologists, such as De la Beche and others, at about 

 seven hundred feet, while it extends through all the northern part of 

 France as far south as Aix-la-Chapelle, thence northward to Denmark 

 and through the south part of England to the Isle of Wight ; and 

 that its outcroppings have been traced from the north of Ireland to 

 the south of France and eastward to the borders of Asia Minor, while 

 a belt of cretaceous deposits extends around the earth just north of 

 the equator, and numerous other chalk regions occur, like that reach- 

 ing from Terra del Fuego to New Granada, in South America, besides 

 those in our own country very extensive in the Western, less so in 

 the Southern States we begin to perceive in what overwhelming 

 quantities these organisms have existed, and what a stupendous work 

 they have performed. The microscopic animals of the Cretaceous may 

 be individually insignificant, but en masse they are certainly far more 

 important than such larger fossils as the mosasaurus, pterodactylus, 

 iguanodon, ichthyosaurus, and species of large fossil turtles of the 

 same age, or the elephant-like mastodons and ponderous, sloth-like 

 megatheriums of more modern date. The microscopic shells, which 

 chalk contains, are 'mostly in a fragmentary condition, yet plenty are 

 entire enough to be readily identified, and the number of different 

 kinds (species) involved was very great, for about three hundred spe- 

 cies have been described. Twenty of these species are still living and 

 more or less actively engaged, along with other living species, in the 

 construction of modern chalk, or the chalk-mud of the Atlantic basin. 

 Here in the bottom of the sea we have chalk in the actual process of 

 formation to-day. It was long since said by Dr. Mantell that " chalk 

 forms such an assemblage of sedimentary deposits as would probably 

 be presented to observation if a mass of the bed of the Atlantic two 

 thousand feet in thickness were elevated above the waters and became 

 dry land ; the only essential difference would be the generic and spe- 

 cific characters of the imbedded animal and vegetable remains," * and 

 * " Wonders of Geology," 1848, vol. vi., p. 305. 



