DISINTEGRATING WORK OF HEAT. 437 



changes in temperature has usually been called exfoliation. Walther, 

 referring to the agency of the sun in producing change of temperature of 

 rocks, has named the process insolation, and of it he gives many instances." 

 The annual ranges of temperature in various parts of the United States 

 run from 30° C, as at San Francisco, to 67° C, as at Moorehead, Minn. 

 Gannett says that in the interior of Alaska the annual range is sometimes as 

 great as 80° G. b Hahn agrees that the annual range of temperature in some 

 regions is as high as 70° to 80° C. c While the seasonal changes of tempera- 

 ture are therefore greater than the diurnal or cyclonic changes, the times of 

 the changes are far greater, and therefore the rates of the changes far less. 

 While the seasonal change undoubtedly produces some effect in disinte- 

 grating the rocks near the surface, probably the more important effect of 

 the seasonal change is at a greater depth than the diurnal or cyclonic 

 changes. At a depth of 1 or 2 meters, or at most several meters, the diurnal 

 or cyclonic effect must be obliterated. Since the seasonal effect some 

 meters below the surface is produced very slowly, probably this effect 

 is not determined by the maximum range of temperature, but by the range 

 of the hottest season of summer compared with the coldest season of winter, 

 for a long period of time is required in order to produce an effect of differ- 

 ential expansion more than a meter or two below the surface. If January be 

 compared with July, the change in temperature in the arctic regions would 

 be from about 45° to 60° C, the change in the temperate regions would be 

 from about 20° to 45° C, and the change in the tropical regions would be 

 from about 10° to 20° C. If the comparison were made of January and 

 February together with July and August together, these numbers would be 

 but little different. Since the annual changes of temperature are small in 

 the tropical regions, are much greater in the temperate regions, and are 

 very great in the arctic regions, it follows that, so far as the annual changes 

 in temperature are concerned, the effect is a minimum in the tropics and a 

 maximum in the arctic regions. The seasonal changes of temperature 

 produce an effect to a depth of 12 to 15 meters. Probably the effect which 

 can be discriminated from the diurnal and cyclonic changes is between 3 

 or 4 to 12 or 15 meters. It seems to me probable that many of the joints 



a Walther, Johannes, Die Denudation in der Wuste unci ihre geologische Bedeutung: Abhandl. 

 Math.-phys. Classe Gesell. Wiss. Leipzig, vol. 16, 1891, pp. 448-453. 



6 Gannett, Henry, The general geography of Alaska: Nat. Geog. Mag., vol. 12, 1901, p. 191. 

 c Hahn, Julius, Handbuch der Klimatology, J. Engelhorn, Stuttgart, 1S83, p. 497. 



