500 A TREATISE ON METAMORPHISM. 



Also the low temperature is unfavorable to chemical action and to life, 

 and transportation follows closely upon disintegration. The result, there- 

 fore, of marked topographic relief is rapid disintegration with comparatively 

 little decomposition. 



Regions of sparse plants and animals. — That sparseness of plants and animals is 

 favorable to disintegration without decomposition has already been shown, 

 since it has been made clear that decomposition is largely dependent upon 

 the action of plants and animals and their by-products. The sparseness of 

 life may be clue to low temperature, low humidity, or other causes. 



Regions near the sea. — Nearness to the sea or to a large river is favorable to 

 disintegration with subordinate decomposition, since the material is trans- 

 ported only a short distance before it is deposited below the Water. But 

 this relation is not so important as one might at first think. The amount 

 of decomposition is far more dependent upon whether the conditions are 

 favorable for this process before the material reaches the larg*e streams than 

 upon distance from the sea. 



When the material once gets permanently below the surface of the 

 water, it passes to a considerable extent from the conditions of the belt of 

 weathering to those of cementation. While oxygen and living organisms 

 may still act upon the material, they are not nearly so effective as under the 

 conditions of the belt of weathering; hence, the process of decomposition is 

 much retarded. This is well illustrated by the material brought down by 

 the Nile and deposited in its lower reaches. According to Judd, the deposits 

 of the Nile delta consist of fresh, unaltered, rounded, angular and subangular 

 grains of the original minerals of the rock, such as the feldspars, hornblende, 

 augite, quartz, etc. This material, he says, has been derived from the desert 

 sands, which lie on either side of the Nile Valley and are swept into it by 

 the wind." As has been seen (pp. 496-498), desert conditions are favorable 

 to disintegration without decomposition; hence the sands contributed to the 

 upper Nile are little decomposed, and the long river journey does not 

 greatly increase the amount of decomposition. 



While decomposition largely ceases when the material is transported 

 by streams, disintegration does not cease, for the particles grind against one 

 another or over the bottom, and thus comminution goes on ; and therefore the 



"Judd, J. W., Report on a series of specimens of the deposits of the Nile delta, obtained by the 

 recent boring operations: Proc. Royal Soc. London, vol. 39, 1885, pp. 213-220. 



