438 A TREATISE ON METAMORPHISM. 



of massive rocks parallel to the surface which may be noted between these 

 limits are the result of seasonal changes in temperature." 



The disintegrating effect of changes of temperature is most marked in 

 those regions in which the precipitation is small for at least a portion of 

 each year. The reason for the great activity of the process during dry 

 periods is plain. On account of the low humidity the sun is very effective. 

 For the same reason the radiation is rapid at night. Consequently there 

 are very rapid changes of temperature, and hence very great exfoliation. 

 These conditions of lack of humidity are ideally illustrated in the desert 

 regions. In such areas it is well known that insolation is very rapid. 6 

 However, probably insolation is not so rapid in those deserts in which there 

 is no precipitation as in those in which there is occasionally abundant 

 precipitation. The conditions for insolation are furnished by the dry periods 

 extending from one to several years. When the very abundant precipita- 

 tion, perhaps in the form of cloud-burst, comes it transports the loosened 

 material away from the parent rock and leaves it bare for further insolation. 

 This condition of affairs is well illustrated in the desert regions of the 

 United States. Also insolation may be very rapid in regions which are 

 considered humid, provided a period of each year is dry. This is well 

 illustrated by Brazil, where there are alternately wet and dry seasons. 

 During a dry season insolation loosens the material; during the wet season 

 the water transports the loosened material to the lower ground, and thus 

 exposes the fresh rocks to insolation during the next dry season. 



In regions in which the precipitation is plentifully distributed over all 

 parts, of the year, insolation is small, for the rocks become covered by 

 vegetation, which prevents the direct action of the sun. In the humid 

 temperate parts of the world are many regions in which insolation is at a 

 minimum, such as the southern Appalachians and the Olympic Mountains. 



a Since the above was put in type Gilbert has said: "In many dome-like granite hills the rock 

 is divided into plates by curved joints approximately parallel to the surface. Some observers call the 

 structure exfoliation; others regard it as an original structure of the granite. Under one view the 

 surface forms determine the structure; under the other the structure determines the surface forms. 

 A study of the High Sierra of California in the summer of 1903 has led me to accept the former view, 

 and to believe that the forms of the parting planes are conditioned by the forms of the topography. 

 As to the cause of the phenomenon the following hypothesis is advanced: Formed deep within the 

 crust, the granite was initially subject to compressive stress, which was balanced by internal expansive 

 stress. As the unloading involved in subsequent denudation reduced the compressive stress, the 

 unbalanced expansive stress caused strains which eventually resulted in exfoliation." Gilbert's view 

 has much in its favor for many of the deeper joints parallel to the surface, but it does not exclude the 

 formation of ruptures as above described. Indeed the two causes may work together. 



''Merrill, Rocks, rock-weathering, and soils, p. 283. Walther, cit., pp. 448-453. 



