158 DYNAMICAL GEOLOGY. 



have this power of boring into stone. Various species also bore into shells 

 or corals. In seven years, Carrara marble, in the sea south of Long Island, 

 became riddled with borings made by a Sponge, the Cliona sulplmrea of 

 Verrill. Termites, or White Ants, and many other insects, especially when 

 in the larval state, the Limnoria among Crustaceans, and the Teredo, related 

 to Pholas, among MoUusks, bore into wood ; and the last is so destructive to 

 ships, piles, and wharves that it is often called the Shipworm or Pileworm. 



3. The tunneling of the earth by small quadrupeds, as the Mole, and 

 by Crustaceans like the Crawfish, sometimes results in the draining of ponds, 

 and the consequent excavation of gullies or gorges by the out-flowing waters. 

 The tunneling of the levees of the Mississippi by Crawfish is one cause 

 of breaks, and thereby of great floods over the country. 



4. Animals using Mollusks and Echinoderms as food make great refuse- 

 heaps, or beds of broken shells. The animals include Man, as well as other 

 species ; and the beds made by Fishes off the coast of Maine, as described 

 by Verrill (who has drawn attention to this mode of making broken shells), 

 are of great extent. They might be taken for beach deposits. The chief 

 enemy of the American Oyster is a Starfish, which spreads its extensile 

 mouth-opening over the young Oyster, and so gets it inside its stomach, and 

 then, as the shell opens, digests the Oyster. 



5. Fungi attack dead plants and animals, and rapidly destroy them. They 

 do it by excreting ferments or poisons, which eat into and destroy the tissues. 

 Living plants often suffer from this cause when in an enfeebled state. 



6. The destruction of the vegetation of a region by insect life, and that 

 of animals by one another, are also of great geological importance. 



ni. THE ATMOSPHERE AS A MECHANICAL AGENT. 



The weight of 100 cubic inches of dry air, with the barometer at 30 inches, 

 and the thermometer at 60° F., is 31 grains ; and hence it is but -g^ as heavy 

 as water (or y^ at 32° F.). The weight of a column of the atmosphere a 

 square inch in area of section, when the barometric pressure is 30 inches, and 

 the temperature 32° F., is 14-7 pounds. On this basis, the total weight of 

 the atmosphere is about llf trillions of pounds (Herschel). In England, an 

 atmosphere of pressure, used as a limit in connection with steam, is 29-905 

 inches Bar. at 32° F., or nearly 14| pounds to the square inch; in France, it 

 is 760 millimeters, or 29*922 English inches, at the same temperature. 



The atmosphere, while rightly called the earth's aerial ocean, is an aerial 

 ocean without a definite upper surface, resting on an ever-disturbing base- 

 ment. It extends not only to a height of 40 miles, but, with increasing 

 tenuity, to at least 200 miles, — meteorites having become luminous at this 

 height as a consequence of the friction of air. An upper limit is supposed 

 to be determined by the equilibrium between the gravitation of the mole- 

 cules of the elements constituting it and the expansive force, decreasing 

 upward, that separates the molecules. 



