532 A TREATISE ON METAMORPHISM. 



in the southern Appalachians, Nicaragua, and Brazil must have taken 

 several or many hundreds of thousands of years. The process of decom- 

 position was of course more rapid in these regions than in the colder 

 northern regions, but even allowing for this the time required for the work 

 accomplished must have been great. 



BATE OF WEATHERING. 



The rate of weathering depends upon all the factors that have here- 

 tofore been given. These factors may be classified under the forces and 

 agents at work in weathering, the materials weathered, the thickness of the 

 belt of weathering, and the stage of weathering. 



FORCES AND AGENTS AT WORK IN WEATHERING. 



The rate of weathering is rapid in proportion as the temperature is 

 high; the humidity is great; erosion by wind, water, and ice is active; and 

 plants and animals are abundant. All these statements have been so fully 

 developed on previous pages that they will not be further amplified here. 



MATERIALS WEATHERED. 

 i 



The rate of weathering as .dependent upon the material involves a con- 

 sideration of the chemical composition of the rocks, the mineral composition, 

 and the closeness of aggregation. 



CHEMICAL COMPOSITION. 



In general the more basic a rock the more rapid the rate of weathering, 

 and the more acid a rock the less rapid the rate of weathering. As to indi- 

 vidual elements, the following statements may be made: The richer a rock 

 is in alkalies the more rapid is the rate of weathering. With a given quan- 

 tity of alkalies, the greater the proportion of sodium the more rapid the 

 alteration, and the greater the amount of potassium the less rapid the altera- 

 tion. The greater the amount of alkaline earths the more rapid is the 

 alteration. With a given quantity of alkaline earths, the greater the pro- 

 portion of calcium the more rapid the alteration, and the greater the propor- 

 tion of magnesium the less rapid the alteration. The greater the amount 

 of iron the more rapid is the alteration. The greater the amount of 

 alumina the less rapid is the alteration. Where the iron and aluminum 



