258 TIME CONSIDERATIONS 



latter case, where physical causes predominate, the loosened 

 particles are removed as fast as formed, and new surfaces for 

 attack are being continually exposed. Moreover, in assuming 

 that rocks decay rapidly where covered by vegetation, we must 

 not overlook the fact that the character of the overlying soil 

 may be such as to be protective rather than otherwise. Thus in 

 glaciated regions it is a well-known fact that the striae on rock 

 surfaces are found best preserved where they have been protected 

 from heat and frost by a mantle of drift, or the compact turf so 

 characteristic of the Northern states. (See further under In- 

 fluence of Forests, p. 266.) Culberson has noted^ that rocks on 

 the southern slopes of hills in southeastern Indiana undergo a 

 more rapid weathering than those on the northern. This he 

 regards as due to the more frequent and more extreme changes 

 in temperature on the south slopes, which in that latitude receive 

 a larger amount of heat from the sun's rays. 



(5) Relative Rapidity of Weathering among Eruptive and 

 Sedimentary Rocks. — As to the relative rapidity of chemical 

 decomposition among eruptive and sedimentary rocks, there 

 can — with the exception of the calcareous varieties — be no 

 question, the eruptives being far the more susceptible. This 

 for reasons which will be at once apparent when we consider 

 their origin. The eruptive rocks result from the comparatively 

 sudden cooling of magmas originating far below the action of 

 atmospheric agencies, and are pushed up and allowed to solidify 

 under conditions which are not at all conducive to chemical 

 equilibrium. They are compounds of elements which have 

 combined according to the conditions under which they tempo- 

 rarily existed, but which undergo continual changes as they 

 become exposed by erosion and other causes. They become, in 

 short, out of harmony with their surroundings, and there are at 

 once set up a series of physical and chemical changes such as- 

 shall result in products more in harmony with existing condi- 

 tions, and hence more stable. These changes, briefly put, are 

 those involved in the weathering processes we have described. 

 Indeed, we may well say that rock weathering and all the seem- 

 ingly endless processes of rock decay and rock consolidation 

 are but stages in the continual efforts being made by these inor- 

 ganic particles to adjust themselves to existing conditions. But 



=^Proe. Incliana Acad, of Science, 1879, p. 167. 



