g4 BEACTIONS. 



A secondary influence is the production and widening of cracks by means of 

 roots and stems, but this is often lost sight of in the greater effects of atmospheric 

 weathering. It is also impossible in many cases to separate the effect of plants 

 and atmosphere in the intimate decomposition of rock surfaces. As a rule, 

 however, the paramount action of the plants is indicated by its localization 

 upon certain surfaces or areas. All pioneers on rocks break down the surface 

 in consequence of their excretion of carbon dioxid or other acids, and produce 

 a fine layer of dust. In the case of lichens and many mosses this layer remains 

 in place, but usually it is carried into cracks and crevices. This slow produc- 

 tion of a thin soil or shallow pocket is reinforced by the decay of the pioneers 

 themselves, which also materially increases the nutrient-content and the water- 

 holding capacity. Here, again, it is almost impossible to separate the two 

 reactions; but this is immaterial, since their effect is the same. The combined 

 effect is to produce areas in which rock herbs can secure a foothold and to 

 increase slowly the water-content and the nutrient-content. 



The reaction through weathering takes place most readily when the rock is 

 sedimentary and soft, especially if it is wet or moist during a large part of the 

 growing-season. In such places, the pioneers are mostly mosses and Kver- 

 worts, often preceded by aJgse. Lichens are much less frequent and are apt 

 to be coUemaceous. Water is abundant, and the effect is chiefly to produce 

 a foothold for herbs, apart from the increase of humus. As a consequence, 

 the pioneer stages are often extremely short, and the rocky surface may be 

 quickly covered with herbaceous or even shrubby vegetation. When the 

 rock is exposed to wind and sun, and especially when it is igneous, biogenous 

 weathering begins with the crustose lichens. The influence is exerted at the 

 contact of thaUus and rock, but the corroding carbon dioxid and other secre- 

 tions act also beyond the margin of the thallus during moist periods. This 

 permits the slow extension of each thallus and the starting of new ones, with 

 the result that the rock surfaces with upward or north to east exposure become 

 completely incrusted. The centers of the older thaUi sooner or later die and 

 begin to break up, leaving an area of greater water-retaining capacity for the 

 invasion of foUose lichens. By their greater size and vigor these extend more 

 rapidly, gradually covering the crustose species and causing them to die as a 

 result of the decrease of water and of light. The size and thickness of the 

 f oliose thallus enable it to retain water better, and thus to enhance its power 

 to weather the surface to greater depths. The surface is usually rough and 

 uneven by reason of folds, soredia, etc., and this helps materially in retaining 

 the water, as well as in providing lodging-places for the spores of mosses. In 

 their turn the foliose thaUi break up at the center and offer a favorable field 

 for the invasion of mosses and, more rarely, of low, matlike herbs. In the 

 weathering of the granites and other hard rocks of the Rocky Mountains such 

 herbs follow the mosses and form the fourth stage. In both stages the amount 

 of soil steadily increases, and with it the amount of water. The disappearance 

 of the mosses is apparently due to the change of light intensity and to the root 

 competition of the herbs. The herbaceous mats form almost ideal areas for 

 the colonization of large herbs and grasses, especially at the center, where 

 they first die and decay (plate 21b). 



(4) Reaction upon windrhorne material. — ^This is the reaction which results 

 in the formation of dunes and sand-hills, and probably also of deposits of 



