PRINCIPLES OF ROCK WE A THE RING 



713 



rocks, subjected to very little rainfall, and hence almost com- 

 pletely bare of vegetation, have, under the blistering heat of 

 the desert sun, weathered down into dome shaped masses, their 

 debris in the form of angular bits of gravel being strewn over 

 the j)lain. Particles of this gravel when compared with those 

 which are a product of chemical agencies are found to differ 

 in that each, however friable, is a complex molecule of quartz, 

 feldspar and mica or whatever may be the mineral composition 

 of the rock from which it derived. Aside from a whitening of 

 the feldspathic constituent, due to the reflection of the light 

 from its parted cleavage planes, scarcely any change has taken 

 place, and indeed it more resembles the finely comminuted 

 material from a rock crusher than a product of natural agencies. 



Owing however to the low conducting power of rocks, disin- 

 tegration from this cause alone can go on to any extent only at 

 the immediate surface, and on flat and level planes where the 

 debris is allowed to accumulate must in time completely cease.' 



It is only on hillsides and slopes or where by the erosive 

 action of running water, or by wind, the debris is gradually 

 removed that such can have any geological significance, 

 although the rate of such disintegration is sufificiently rapid in 

 exposed places to be of serious consequence in stone used for 

 architectural application. (See further under Action of Ice.) 



'Observations by Forbes (Trans. Royal Society of Edinburgli, Vol. XVI, 1849), 

 showed that at depths of not above twenty-five feet the mean annual temperature was 

 greater than near the surface, these results being confirmatory of those obtained by 

 QuETELET at Brussels. The following tables from Forbes' paper show maximum 

 range of temperatures at varying depths and also the depths at which the annual 

 range is reduced to 0°. 01 centigrade. 



I. SHOWING RANGE OF TEMPERATURES FAHR. 



