CONDITIONS AFFECTING ROCK DECAY 323 



Even where the moisture is abundant and the temperature is suffi- 

 ciently high for tlie growth of an abundant vegetation, unless the con- 

 ditions are also favorable for the decay of that vegetation they are not 

 favorable for rock weathering. This is seen in the extremely luxuriant 

 forests of the North Pacific coast, where the successive generations of 

 forests grow on the remains of their predecessors. The conditions are 

 here favorable for the preservation of vegetable remains in the form of 

 peat, and rock decay is practically absent. It appears, therefore, that 

 an essential condition for rapid rock weathering is the rapid decay of 

 abundant vegetable matter, and this leads to the conclusion that the most 

 efficient factor in the weathering pi-ocess is the presence of the complex 

 organic acids which are derived from the decay of vegetation. 



Effect of chemical composition. — The depth to which the rock decay has 

 gone and the character of the products depends in a considerable measure 

 on the chemical composition of the rock, on its original structure, and 

 on the subsequent alterations which it has undergone, such as fracturing 

 in the process of consolidation or by subsequent dynamic disturbances. 



But few rocks are found in this region which are not either wholly or 

 in large part composed of material of volcanic origin ; hence their chem- 

 ical composition does not present so wide a range as is usually found 

 among sedimentary and igneous rocks. A few are apparently the pro- 

 ducts of thermal springs, and the composition of these is perhaps the 

 best suited of any to resist the process of rock decay. Examples of rocks 

 of this origin are found in the small hill opposite San Francisco, a short 

 distance east of the lake, and also at Chorrera, on the Aguas Muertas. 

 These are composed chiefly of silica, which is the mineral least acted on 

 by the processes to which rock decay is chiefly due. 



The Machuca sandstone, as already explained, contains a large pro- 

 portion of feldspathic minerals, as well as iron sulphide and carbonate 

 of lime ; hence it is peculiarly susceptible to hydration, oxidation, and 

 solution. 



The igneous rocks belong to the basic and intermediate classes, and 

 hence contain a large proportion of the lime-soda feldspars and the ferro- 

 magnesium minerals. Both of these groups of minerals are especially 

 liable to alteration. Quartz, on the other hand, is relatively scarce. 

 There are in the region no quartzites and argillites, the two classes of 

 rocks which are especially indifferent to the action of the weathering 

 processes. 



Certain beds associated with the lavas are composed of fine volcanic 

 ash, in which the constituent particles had never acquired a crystalline 

 structure, but were entirely glassy. These are pferhaps the most readily 

 altered rocks in the region, and wherever they have been encountered, 



