MODULES. 71 



results will clearl^^ be those discussed above. Au angular mass of such a 

 rock will yield to the decomposing agencies directly as S/V, or in an 

 inverse ratio to its radius of curvature, and, if the mass is homogeneous, it 

 will gradually be reduced to the spherical form. It thus becomes evident 

 that angular blocks of basalt attacked by sulphuric acid or other corrosive 

 fluids tend to the spherical form, not because of any variation in the internal 

 structure, but, on the contrary, because tliey are substantially homogeneous. 



The rocks which do not weather or decompose to rounded masses are 

 the more permeable class. Thus, in the Washoe district dense andesites and 

 basalts tend to the spherical form, while tufaceous masses and porous rocks 

 decompose with tolerable uniformitv throughout. In terms of the mathe- 

 matical discussion for the latter, r <C 1. Just so along the quicksilver belt: 

 dense rocks undergoing serpentinization show rounded nodules of inialtered 

 or slightly altered material, while more permeable masses are gradually 

 changed to serpentine throughout. 



It may not be amiss to note that the depth to which a rock will be at- 

 tacked by any decomposing fluid depends somewhat upon the nature of the 

 fluid. If the reaction between the liquid and the solid is a rapid one the 

 liquid will become substantialh' saturated comparatively near the surface, 

 while, if the reaction is feeble and slow, the fluid will penetrate to a greater 

 depth before losing its corrosive power. Complex cases sometimes result 

 from the co existence of various reactions, each leading to a particidarkind 

 of decomposition. 



Application to pebbles — It is evideut tluit the principles applied in the fore- 

 going discussion are not limited to the action of fluids. Any disintegrating 

 agency acting uniformly on the surface of an angular body or acting suc- 

 cessively on all points of its surface will be governed by similar laws. 

 Consider a fragment of rock in a stream bed or on a beach. It suflPers 

 frequent impacts from other bodies of similar averag-e size and composition. 

 Each of these impacts disintegrates the mass at the small surface of contact 

 to a certain average depth, and these impacts are repeated in indefinite 

 number on all portions of its surface. The result must be the same as if 

 the rock fragment were subjected to a disintegrating action simultaneously 

 at all points of its surface, niid, just as in the case of a solvent fluid, the 



