644 GEOLOGY. 



to age, the climate of the earth must apparently vary accordingly, and 

 on this is built one of the hypotheses of climatic variation subsequently 

 to be considered. We shall find that there have been great changes in 

 the climate of the earth during its history. There is good evidence of 

 former glaciation, not only in the northern United States and in England, 

 Germany, and central Russia, but in India, Australia, and South Africa. 

 At other times, figs and magnolias grew in Greenland and Spitzbergen, 

 and corals flourished in the Arctic seas. There is good evidence of arid 

 periods where humidity now prevails, and of humid periods where 

 aridity now prevails. It is not assumed that the influence of organic 

 action on the atmosphere has been the sole, or perhaps even the main, 

 cause of these great climatic changes, but it is believed that it has been 

 an important contributing factor. It is even possible that the climate 

 of the future is much dependent on the agency of man, as implied above, 

 however little ground there may be to suppose that he will, with altru- 

 istic purpose, control his action with a view to its bearing on the gener- 

 ations that may live tens of thousands of years hence. 



(2) Aid and hindrance to inorganic action. 



The promotion of disintegration. — While the influence of organic 

 action on the hthosphere is quite superficial, and far less radical than 

 that on the atmosphere, it is still important. Plants promote both 

 disintegration and disaggregation under certain conditions, and hinder 

 them under others, as already set forth. Chemical action of a decom- 

 posing and solvent nature takes place in connection with the roots of 

 plants, while their growth sometimes rends rocks into whose crevices 

 they have insinuated themselves. The acids and other products of 

 organic growth and of organic decomposition attack some of the con- 

 stituents of the rocks and contribute to their solution and disintegration. 

 On the other hand, organic matter entrapped in the sediments, and so 

 introduced into the strata at various depths, often acts as a reducing 

 agency, causing the deposit of substances carried in solution in the under- 

 ground-waters. Ores are sometimes thus formed, as explained in the 

 discussion of ore-deposits (p. 476). Organic action on the whole promotes 

 solution and disintegration at the surface, and prepares the way for 

 deposition below. 



Protection against erosion. — Another important function of vegeta- 



