DYNAMICAL GEOLOGY. 239 



variations of eccentricity could affect climate to such an extent, 

 and on the other, he thinks CrolPs whole chain of argument 

 valueless, since, excellent as it is, astronomy has Jiot yet ascer- 

 tained with any security that there have been periods of very 

 great eccentricity of the orbit. Poisson (1837) suggested that 

 climatic variations might result from movement of the solar 

 system through warmer and colder portions of space; other 

 authors have suggested changes in the obliquity of the ecliptic 

 or a shifting of the earth's axis as possible causes of variation, 

 but science has not yet arrived at any generally accepted 

 solution of the difficult climatic problem of the Ice Age. 



D. Geological Action of Organisms. Scientific research has 

 abundantly shown how subtle is the chemistry of life, and how 

 important and complex is the part played by the organic world 

 in the economy of nature. 



Plants and animals abstract from the atmosphere, from the 

 soil and the rocks, certain inorganic constituents which enter 

 into new chemical combinations in the active tissues of the 

 living organisms, and are partly assimilated, partly returned in 

 altered form to the atmosphere and the ground. 



Animal creation thus serves as an intermediary between the 

 atmosphere and the earth's surface, utilising and metabolising 

 matter derived from both, and effecting transferences from the 

 one to the other. 



The present action of living organisms upon the earth's 

 surface is therefore partially to destroy, partially to renew and 

 enrich it; and similar functions were fulfilled by living organ- 

 isms in past ages. But more important for geology than the 

 changes effected by metabolism and mineral decomposition 

 is the consideration of the additions made to rock-deposits by 

 the accumulation of organic remains. 



The destructive effects of plant-growth are produced in virtue 

 both of chemical and mechanical agencies. When plants 

 decay, organic acids develop, and, as Bischof and more 

 recently Julien have shown, these have a strong solvent and 

 oxidising action upon the surrounding mineral matter. More 

 especially when combined with water they promote rapid 

 decomposition of the rocks, and their disintegrating action, 

 productive of soil, may be traced to considerable depths below 

 the surface. The roots of plants as they penetrate downward 

 through the rock-fissures exert a certain mechanical force upon 



