DISINTEGRATING WORK OF HEAT. 439 



Topography is also an important element in insolation. Where the 

 slopes are steep gravity pnlls off the flakes formed, and new surfaces are 

 exposed to the process. Also theoretically the greater the elevation the 

 more the insolating effect, for the sun does uot have to pass through so 

 much of the atmosphere and should be more effective in heating, and the 

 heat gained in the day should be more rapidly radiated at night. Thus 

 one would expect greater changes in the temperature of the rocks at high 

 elevations than at low elevations, but I am not aware that observations 

 have been made on this point. 



Rocks vary greatly in their power to resist the effect of insolation. 

 Sandstones are refractory ; limestones and shales are somewhat less so ; crys- 

 talline rocks, especially those having a large amount of readily cleavable 

 feldspar, are most affected. I suspect the cause of the varying resistance 

 of rocks is a function of their porosity. In proportion as the rocks have 

 many pores, such as sandstones, the expansion and contraction is taken up 

 between the grains, and therefore rupture is not likely to take place. 

 Where, however, the rock is a solid continuous mass, with little or no pore 

 space, when the temperature of the surface is higher than that of the rock 

 below, the expansion can be taken up' only by elasticity. When the stresses 

 accumulate beyond the elastic limit, ruptures are produced. (See p. 434.) 



Frequently insolation results in burying' the crystalline rocks, except 

 the sharpest peaks, beneath debris. Regions which well illustrate the dis- 

 integration largely due to change of temperature are southern California and 

 the peninsula of Lower California, described by Merrill" and b} 7 McGee; 6 

 Egypt, described by Walther;" and Brazil, described by Brainier. d 



Where the rocks become covered with flakes and spalls, this of course 

 protects the solid rock below, and thus stays the process of insolation. 



Disintegration as a result of insolation can be certainly discriminated 

 in the low-lying regions of the Tropics, in which the temperature never falls 

 below 0° C. But in all areas in which the temperature frequently falls 

 below 0° C. the disintegrating effect of insolation is conjoint with that of 

 freezing and thawing, next considered. 



"Merrill, George P., Sketch of Lower California: Bull. Geol. Soc. America, vol. 5, 1894, p. 499. 

 » McGee, W J, The formation of arkose: Science, new ser. , vol. 4, 1896, pp. 962-963. 

 cWalther, cit., pp. 347-569. 



^Branner, J. C, Decomposition of rocks of Brazil: Bull. Geol. Soc. America, vol. 7, 1896, 

 pp. 281-294. 



