240 THE PHYSICAL MANIFESTATIONS OF WEATHEEINa 



merely the insoluble impurities contained by the original lime- 

 stone, but also the mechanically disintegrated particles washed 

 in from the hills on either hand. This indeed may be consid- 

 ered the history of the fertile Shenandoah valley of Virginia, 

 famous alike for soft contours, beautiful scenery, and the exu- 

 berant fertility of its soils. 



In cases where thinly bedded rocks lie sharply inclined, it 

 nearly always happens that certain layers decompose more read- 

 ily than others. There may thus arise strikingly ragged saw- 

 tooth contours, the more enduring layers standing out in sharply 

 serrate or wall-like masses, while the softer give way and be- 

 come obscured by their own debris. 



"When stratified rocks lie nearly or quite horizontally, much 

 must depend upon the character as regards permeability, etc., 

 of the upper layers, since these may so protect the lower lying 

 as to retard or quite stop further disintegration. Further than 

 this, an easy and rapidly disintegrating superficial layer may 

 yield a residual clay so impervious as to protect the underlying 

 rocks as securely as a mass of rock itself, or so hard and tough 

 as to put a stop to purely mechanical erosion, as in the case of 

 the laterite beds of central India. 



(6) Induration on Exposure. — Many rocks, instead of becom- 

 ing disintegrated on exposure, undergo a kind of induration 

 upon the exposed surfaces. This is particularly the case with 

 some siliceous sandstones. The water with which the stone is 

 permeated holds in solution certain constituents, as silica, car- 

 bonate of lime, or iron oxides. When the rock is so situated 

 that this ''quarry water," as it is popularly called, is brought 

 to the surface and evaporated, it binds together the granules 

 composing the stone, forming thus a more or less superficial 

 coating of a more enduring nature. The induration sometimes 

 takes place so rapidly that even an exposure of but a few months 

 is sufficient to produce very marked results on freshly broken 

 surfaces. This peculiarity of certain classes of rocks has long 

 been known to quarrymen and stone workers, who recognize 

 the fact that a well-seasoned stone yields much less readily under 

 the chisel than one that is newly quarried.^ 



A somewhat similar induration, due to purely superficial 

 causes, has been described^ by Dr, M. E. Wadsworth, as taking 



^See Stones for Building and Decoration, p. 415. 



^ Proc. Boston Soe. of Natural History, Vol. XXII, 1883, p. 202. 



