DEPTH OF WEATHERING. 529 



syenites. The nepheline-syenite of the Wausau district of central Wis- 

 consin has a peculiar, deeply pitted appearance, due to the complete 

 solution of the nephelites at the surface. 



A further case of irregular surface is furnished by the cherty lime- 

 stones; the calcite is dissolved more rapidly than the quartz, and thus the 

 weathered surface presents a series of ridges and hollows, giving a very 

 rough, ridgy surface. 



DEPTH AND DEGREE OF WEATHERING. 



The belt of weathering has already been defined as extending from 

 the surface to the level of ground water, and locally to some distance below 

 the level of ground water. (See ]3p. 409-411.) Furthermore, it has been 

 seen that the level of ground water varies from to more than 300 meters 

 below the surface. 



Whatever the thickness of the belt of weathering, the conditions 

 throughout this belt are favorable for oxidation, carbonation, hydration, 

 and solution, and therefore for the chemical reactions of decomposition. 

 But the conditions are far more favorable for decomposition at the surface 

 than at the lower part of the belt of weathering. Disintegration is much 

 more limited in extent. It is at a maximum at the surface, but decreases 

 rapidly with depth, and at 10 meters below the surface is scarcely appre- 

 ciable. While it is clear that reactions of weathering extend throughout 

 the belt as defined, it is equally certain that the processes are at their maxi- 

 mum efficiency at or near the surface, and decrease in efficiency as depth 

 increases. But in some cases the processes are very important to the 

 bottom of the belt, even where of very considerable thickness. This is 

 illustrated by limestone regions, in which minor openings or even extensive 

 caves may form abundantly quite to the level of ground water, although 

 this may be 100 or 200 or more meters below the surface. 



As to degree or stage of weathering, rocks are ordinarily spoken of as 

 "weathered" where they are very appreciably affected by the peculiar weath- 

 ering processes. As already explained, there are all gradations, from no 

 weathering to complete weathering. Both extremes are very rare, although 

 it is common for geologists to speak of "complete weathering" without telling 

 what they mean thereby. In regions where the conditions for weathering 



MON XLVII — 04 34 



