THE GEOLOGIC FUNCTIONS OF LIFE. 645 



tion is the protection of the land surface against erosion, as already 

 noted in the discussion of erosion. A mantle of grass, especially if it 

 forms a turf, or a carpet of leaves protected by bush and forest, 

 greatly retards surface wash. It does this not only because it directly 

 covers the soil, but because it holds back the run-off and tends to pre- 

 vent those violent floods which give to erosion its greatest intensity. 

 There is a marked difference between the erosive work which a given 

 amount of water will do if, in the one case, it runs off gradually, and in 

 the other, precipitately. By way of offset, it is to be noted that the 

 disintegrating action of vegetation prepares- the rock material for easy 

 erosion, and to this extent helps in its removal by the drainage ; but on 

 the average this is greatly overbalanced by the protection afforded 

 by the vegetal covering, though this is not true in every instance. 



The influence of land vegetation on the character of the sediments. — 

 The presence or absence of a vegetal covering influences the kind of 

 deposit which is derived from the land, particularly if the surface be 

 occupied by crystalline rocks. If the surface be well clothed with vege- 

 tation, the crystals of the complex silicates, such as the feldspars, micas, 

 and ferromagnesian minerals, are usually disintegrated into clayey 

 products before they are removed, so that, when borne away and 

 deposited, the result is common shale.. Concurrently, the relatively 

 undecomposable quartz-grains are rounded into sand, and deposited 

 as common quartzose sandstone, while the calcareous material is borne 

 away in solution and deposited as hmestone. But if the surface be 

 bare of vegetation, the crystalline rocks are usually disaggregated heiom 

 they are decomposed, for destructive action works best at the junctions 

 of crystals, and along cleavage lines, and hence the crystals are usually 

 separated from one another before they are fully decomposed. In the 

 absence of a covering to hold them in place until they are decomposed, 

 they are apt to be washed away, and the resulting deposit consists in 

 considerable part of grains of feldspar, mica, hornblende, and other 

 minerals, which do not usually occur in well-decomposed sediments. 

 The deposits are, therefore, of the nature of arkose, if the original rocks 

 are granitic, or of the nature of wacke, as the term is used in this book, 

 if they are of the basic type. On this is based the inference that a 

 vegetal covering of the land extended as far back in the history of the 

 earth as clay shales, quartzose sandstones, and limestones form the 

 prevailing sediments. 



