RATE OF WEATHERING. 533 



mutually replace each other, the greater the proportion of aluminum the 

 more rapid the alteration. The greater the amount of silica the less rapid 

 is the alteration. 



MINERAL COMPOSITION. 



The readiness of the alteration of minerals is largely a direct conse- 

 quence of the character and the chemical composition. With a given 

 chemical composition, the greater the proportion of glass the more rapid 

 the alteration, and the greater the proportion of crystallized minerals the 

 slower the alteration. Among the crystallized minerals, the greater the 

 proportion of the easily decomposed minerals the more rapid the alteration. 



The order of decomposition of the common anhydrous minerals from 

 the easiest to the hardest, as given on pages 518-521, is (1) nephelites and 

 leucites, (2) olivines, (3) pyroxenes and amphiboles, (4) biotite-muscovite 

 group, (5) feldspars, (6) garnets, staurolite, tourmaline, andalusite, cyanite, 

 and (7) hematite, magnetite, and quartz. 



STATE OF AGGREGATION. 



Fine-grained rocks weather less rapidly than coarse-grained ones. 

 This is a consequence of the closer interlocking of the mineral particles of 

 the fine-grained rocks, and of the fact that the differential expansion and 

 contraction by change of temperature is less with fine particles than with 

 coarse particles. The more open the aggregation the more rapid the alter- 

 ation; the closer the aggregation the less rapid the alteration. This follows 

 from the fact that openings give ready access to the agents of alteration, and 

 the border of every opening is a place of attack for the chemical agents, and 

 therefore all classes of openings promote decomposition. The openings in 

 rocks are divided into pore spaces between the mineral particles, sheet open- 

 ings, and irregular openings (see p. 129). Pore spaces are generally in the 

 mechanical sediments, the lavas, and the tuffs; therefore mechanical sedi- 

 ments, lavas, and tuffs alter much more rapidly than the plutonic igneous 

 rocks and the compact metamorphic rocks. Sheet openings are of various 

 kinds, such as faults, joints, bedding partings, and fissility. All these open- 

 ings serve as channels through which the waters may readily pass and extend 

 the decomposing effect. In general, along a fault or a joint, decomposition 

 extends more rapidly downward than in the adjacent solid rock. Where 



