608 



DYNAMICAL GEOLOGY. 



of massive and jointed rocks are thickly covered with huge blocks, 

 looking like transported bowlders, which are the results of this kind 

 of upturning. The opening of fissures by roots also gives access to 

 moisture, and thus contributes further to rock destruction. 



2. Boring animals, like the saxicavous Mollusks, make holes often 

 as large as the finger, and sometimes larger, in limestone and other 

 rocks, along some sea-shores. Species of Saxicava, Pholas, Petricola, 

 Lithodomus, Gastrochcena, and even some Gasteropods, Barnacles, 

 Annelids, Echini, and Sponges, have this power of boring into stone. 

 Various species also bore into shells or corals ; and the animal of the 

 shell is thus often killed, and the crumbling of the shells and corals 

 is much hastened. The Termites and many other insects, especially 

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

 among Mollusks, bore into wood. 



3. The tunnelling of the earth done by small quadrupeds, as the 

 Mole, and by Crustaceans like the Craw-fish, sometimes results in the 

 draining of ponds, and the consequent excavation of gullies or gorges 

 by the outflowing waters. The tunnelling of the levees of the Mis- 

 sissippi by Craw-fish is one prominent cause of breaks, and thereby 

 of great floods over the country. 



4. The decay of vegetation about rocks often produces carbonic 

 acid or different vegetable acids, which become absorbed by the 

 moisture of the soil, and thus penetrate the crevices of rocks and pro- 

 mote their decomposition. This is properly one of the chemical 

 effects of life. 



5. 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. 



6. Fungi attack dead plants and animals, and rapidly destroy thenu 



7. The destruction also of the vegetation of a region by insect life,, 

 and that of animals by one another, are of geological importance. 



4. Contributions to Rock Formations. 



The capability, on the part of Life, of contributing to the material of 

 rocks, depends on several considerations, of which the following are 

 the more prominent : — 



1. The conditions favoring or limiting growth and distribution, — 

 that is, the laws of geographical distribution of living species. 



2. The nature of different organic products, and the fitness of the 

 species affording them for making fossils or rocks. 



