lyo - STUDIES FOR STUDENTS 



The sharpness of the ridges of the Sierra Madre, and the roughness of the hard 

 granitic surfaces, contrast ^sharply with the rounded ranges formed by the 

 "rotten" granites of the Atlantic slope, where sound, unaltered rock can some- 

 times not be found at a less depth than forty feet; while at the foot of the Sierra 

 Madre ridges, thick beds of sharp, fresh granitic sand, too open and pervious 

 to serve as soils, cover the upper slopes and the "washes" of the streams, causing 

 the latter to sink out of sight. A general discussion of the kinds of soils formed 

 from the various'rocks must, therefore, take these differences into due consideration. 



The lack of decomposition and the dominance of disintegration 

 in desert regions, giving rise to fresh and unweathered sands, have 

 also been emphasized by Walther, who shoM^s that insolation breaks 

 up crystalline rocks into a rubbish of crystals scarcely altered chemi- 

 cally.^ Merrill and others have abundantly confirmed this as a 

 principle dominating the production of rock waste in arid climates. 



OLD TOPOGRAPHY AND VARIOUS CLIMATES 



Regions topographically old are of lessened importance from the 

 view-point of the mechanical sediments of running water, but little 

 waste being contributed to the deltas and the seas and the conditions 

 for limestone formation approaching close to the shores. In the 

 old age of the humid region the blanket of decomposed rock becomes 

 universal and increased in thickness, giving rise upon erosion to 

 fine-grained and well-decomposed silts. In the arid regions, on the 

 contrary, the waste in the rock basins diminishes in thickness through 

 wind and water erosion and the desert becomes finally covered with 

 a thin gravelly or sandy mantle stiU characterized by lack of decom- 

 position. In the old age of arid regions, as shown by Passarge, wind 

 erosion becomes increasingly more important than water erosion, 

 since the water loses its force upon the flat desert surface, while 

 the action of the wind does not diminish in intensity. The products 

 of erosion in old age, therefore, are chiefly wind-borne loess and dune 

 sand, possessing distinctive qualities and a different distribution 

 from sediments of fluvial and pluvial origin. 



Relations of Temperature and Topography to Erosion . 



EFFECTS of TEMPERATURE VARIATIONS ON VEGETATION AND 

 SOIL RETENTION 



Besides the relations dependent upon rainfall, those dependent 

 upon temperature may also be considered. In general, increased cold 

 I Einleitung in die Geologic, 1893-94, pp. 546-47. 



