THE ATTACK OF THE WEATHER 



151 



The water which percolates downward upon the joints, finds 

 its way laterally along the parting planes, and so subjects the en- 

 tire surface of each block to simultaneous attack by its reagents. 

 Though all parts of the surface of each block are alike subject to 

 attack, it is the angles and the edges which are most vigorously 

 acted upon. In the narrow crevices the solutions move but slug- 

 gishly, and as they are soon impoverished of their reagents in the 

 attack upon the rock, fresh solution can reach the middle of the 

 faces from relatively few directions. The edges are at the same 

 time being reached from many more directions, and the corners 

 from a still larger number. 



The minerals newly formed by these chemical processes of 

 hydration and carbonization are notably lighter, and hence more 

 bulky than the minerals from whose constituents they have been 

 largely formed. Strains are thus set up which tend to separate 

 the bulkier new material from the core of unaltered rock below. 

 As the process continues, distinct channels for the moving waters 

 are developed favorable to action at the edges and corners of the 

 blocks. Eventually, the squared block is by this process trans- 

 formed into a spheroidal core of still unaltered rock wrapped in 

 layers of decomposed material, like the outer wrappings of an onion. 

 These in turn are usually imbedded in more thoroughly disinte- 

 grated material from which 

 the shell structure has dis- 

 appeared (Fig. 156). 



Exfoliation or scaling. A 

 fact of much importance to 

 geologists, but one far too 

 often overlooked, is that rocks 

 are but poor conductors for 

 heat. It results from this 

 that in the bright sun of a 

 summer's day a thin skin, as 



FIG. 



156. Spheroidal weathering of an 

 igneous rock. 



it were, upon the rock surface may be heated to a relatively high 

 temperature, although the layer immediately below it is prac- 

 tically unaffected. The consequent expansion of the surface layer 

 causes stresses that tend to scale it off from the layer below, 

 which, uncovered in its turn, develops new strains of the same 

 sort. This process of exfoliation acquires exceptional importance 



