498 A TREATISE ON METAMORPHISM. 



The soils of the arid regions investigated by Hilgard therefore contain 

 from four to fourteen times more of the elements potassium, sodium, 

 calcium, and magnesium, than do those of the humid regions. This 

 difference is doubtless due in part to lack of decomposition of the original 

 compounds; but it is also due in part to the fact that the decomposed 

 materials of the arid regions have not been leached as they have been in 

 the humid regions. As shown in another place (pp. 541-543), the soluble 

 constituents resulting from the decomposition of the original minerals are 

 largely retained. 



In conclusion, we see that aridity or low humidity is generally favorable 

 to mechanical rather than to chemical work. In arid regions the process of 

 disintegration is rapid, and that of decomposition and solution is slow. The. 

 total rate of weathering in arid regions, including mechanical and chemical 

 work, is probably, on the average, less rapid than in humid regions. 



Regions of high latitude. — First, regions of high latitude are those of low 

 temperature. It has already been seen that the disrupting effect of change 

 from water to ice is an important factor in the disintegration of rocks. This 

 process is applicable only to those regions in which the temperature falls 

 below 0° C, and to such regions for only those parts of the year in which 

 the temperature alternately passes above and below the zero mark. These 

 conditions at low altitude do not occur at all in the Tropics. They occur 

 in winter only in the parts of the temperate zones near the Tropics. They 

 occur in spring and autumn in the cooler parts of the temperate zones and 

 in the frigid zones. However, in passing from low to high latitude the 

 length of time during each year in which the conditions are those of 

 alternate freezing and thawing increases; also the range above and below 

 zero increases. Hence the disintegration due to freezing and thawing 

 increases as the latitude increases from those parts of the temperate zones 

 near the Tropics to the far northern and southern lands. Second, regions 

 of high latitude are those of extensive glaciation. The glaciers are most 

 powerful disintegrating agents, producing de'bris ranging in size from great 

 bowlders to the minute particles which whiten the waters issuing from 

 them, and all this with scarcely a trace of decomposition. Third, in regions 

 of high latitude there are great changes in temperature which work by 

 differential expansion, as explained on pages 438-439, and thus help to 

 disintegrate the rocks. Absence of decomposition in connection with disin- 



