606 DYNAMICAL GEOLOGY. 



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

 the size of the finger, or larger, in limestone and other rocks along 

 some sea-shores. Species of Saxicava, Pholas, Gastrochcena, and even 

 some Snails, Barnacles, and Echini, have this power of boring into 

 stone. 



The Teredo, Termites, and many insects, especially in the larval 

 state, bore into wood. 



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

 mole, sometimes results in the draining of ponds, and the conse- 

 quent excavation of gullies or gorges by the outflowing waters. 



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 

 promote their decomposition. This is properly one of the chemical 

 effects of life. 



4. Contributions to Kock-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. 



After discussing these subjects, some of the methods of contri- 

 buting to rock-formations are mentioned under the heads, — 



3. Methods of fossilization and concretion. 



4. Examples of the formation of strata through the agency of 

 life. 



1. Geographical Distribution. 



The subject of the geographical distribution of plants and ani- 

 mals, though highly important in this connection, cannot be pro- 

 perly treated in a brief chapter ; and the student is therefore re- 

 ferred to treatises on this branch of science. Its general principles 

 and bearing are all that can here be explained. 



The distribution of terrestrial plants and animals is limited by 

 different causes. 



1. Climate. — The temperature to which each is adapted in its 

 nature determines, within certain limits, its position in the zones 

 between the equator and the poles, and also, under any zone, 

 its special altitude between the level of the sea and the height of 



