612 DYNAMICAL GEOLOGY. 



and in others, to oxyd of iron and pyrites ; and more rarely to fluor spar, heavy 

 spar, or phosphate of lime. 



The remains of organisms have very frequently been ground up 

 by the action of waves or by currents of water, and thus reduced 

 to a calcareous earth, — the concretion of which has made lime- 

 stones. 



When the fossils are minute, like Ehizopods and Diatoms, the 

 simple concretion of the shells will make a solid rock, as in the 

 case of chalk and flint (p. 488). 



Ehrenberg estimates that about 18,000 cubic feet of siliceous organisms annu- 

 ally form in the harbor of Wismar in the Baltic; and he has also found that 

 similar accumulations are going on in the mud of American and other harbors. 



The bed of Ehizopods accumulating in the North Atlantic, mentioned on page 

 488, contains, according to Huxley, about 85 per cent, of these calcareous shells, 

 mostly of the genus Globigerina, besides some siliceous Diatoms: it has probably 

 a breadth (between Ireland and Newfoundland) of 1300 miles; and, as a similar 

 bottom was found by Captain Dayman near the Azores, the bed has been sup- 

 posed to extend southward at least 600 miles. Ehrenberg found, in a specimen 

 examined by him, 85 species, of calcareous Rhizopods, 16 of Polycystines, and 

 17 of Diatoms, with only a few arenaceous grains not of organic origin. 



Off the Atlantic coast, from Florida north, between depths of 90 and 1500 

 feet, the bottom consists half or more of Rhizopod shells ; and at greater depths, 

 even beneath the Gulf Stream, to 6000 feet (as observed in lat. 28° 24', long. 

 79° 13'), almost solely of them. (Pourtales.) 



The siliceous shells of the microscopic Polycystines have been found not only 

 in the frigid Sea of Kamtchatka (p. 488 ; see Amer. Jour. Sci. [2] xxii. pi. 1, 

 for figures) and the North Atlantic, but also in the South Pacific, on both coasts 

 of the Atlantic, in the Mediterranean, and, within the tropics, at Barbadoes in 

 the West Indies and the Nicobar Islands in the East Indies. Ehrenberg has 

 named 282 species from a marl-like deposit at Barbadoes, considered as Ter- 

 tiary, and 100 species from the Nicobar Islands, part of them identical with those 

 of Barbadoes. 



But when the fossils are comparatively large, as with ordinary 

 corals and shells, the intervals between them must be filled with 

 earth of some kind, derived from the wearing action of the waters. 

 It may be the mud or detritus from rivers or from wave-action along 

 sea-shores. But when calcareous, it has evidently come from the 

 5 wear of the shells, corals, or crinoids themselves ; and hence any 

 limestone rock made up of shells, corals, or crinoids which has the 

 interstices thus filled in with limestone bears conclusive evidence 

 in itself that it has not been formed in the deep ocean, but within 

 the reach of current or wave action. Ehizopods make the only 

 solid deep-water limestones. 



The kinds of limestone made through the agency of life include 

 soft marl or calcareous earth, chalk, compact limestone, sometimes 



