Page:Popular Science Monthly Volume 15.djvu/788

 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 formation of cretaceous deposits, but the great bulk was undoubtedly of rhizopod 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 hundred 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 estimated by reliable geologists, such as De la Bêche 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 reaching 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 species 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 specific characters of the imbedded animal and vegetable remains," and