162 THE PEIjSTCIPLES INVOLVED IN BOCK-WEATHEBING 



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 resem- 

 bles the finely comminuted material from a rock-crusher than 

 a product of natural agencies. 



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

 integration from this cause alone can go on to any extent only 

 at the immediate surface, and on flat and level plains, 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 re- 

 moved, that such can have any geological significance, although 

 the rate of such disintegration is sufficiently rapid in exposed 

 places to be of serious consequence in stone used for architectural 

 application. (See further on p. 177, Action of Ice.) 



^ Obsexvations on soil temperatures made at tlie Orono, Maine, Experi- 

 ment Station showed that the mean daily range of temperatures from 

 April to October, at a depth below the surface of 1 inch, was 5.62° ; at a 

 depth of 3 inches, 5.26°; at 6 inches, 1.9°; and at 9 inches, 1.18°; and at 

 12 inches very slight. At the depth of 1 inch the temperatre was lower 

 than that of the air by 2.4°; at 3 inches by 2.11°; at 6 inches by 3.16°; at 

 9 inches by 3.94° ; at 12 inches by 4.18° ; at 24 inches by 5.78° ; and at 36 

 inches by 7.10°. 



The following table, compiled by Forbes (Trans. Boyal Society of Edin- 

 burgh, Vol. XVI, 1849), from observations made near Edinburgh, Scotland, 

 during 1841-42, shows the range of earth temperatures at varying depths 

 in soil, sandstone, and trap rock. 



Dbpth 



Teap Eook 



Sand of Gardeit 



Ckaiglhith Sandstone 



Max. 



Min. 



Eange 



Max. 



Min. 



Eange 



Max. 



Mm. 



Eange 



3 feet . . . 

 6 feet . • . 

 12 feet , . 

 24 feet . . 



52.85° 



51.07 



49.00 



47.50 



38.88° 

 40.78 

 44 20 

 46.12 



13.97° 

 10.29 

 4.80 

 1.38 



64 60° 

 52.95 

 50.40 

 48.10 



37.85* 

 39.56 

 43.50 

 46.10 



17.65° 

 13.40 

 6.90 

 2.00 



53.16° 

 51.90 

 60.30 

 48,25 



38.25° 

 38.95 

 41.60 

 44.35 



14.90^ 

 12.95 

 8.70 

 3.90 



It has been shown that the thermal conductivity of rocks varies in direc- 

 tion according to their structure, being greatest in the direction of their 

 schistosity, where such exists. In massive, homogeneous rocks the con- 

 ductivity is the same in all directions. In finely fissile rocks, on the other 

 hand, it may be four times as great in the direction of their fissility as at 

 right angles thereto. 



