
334 ) DYNAMICAL GEOLOGY. [Book TIL. — 
similar transformations take place in the natural silicates of rocks _ 
seems in the highest degree probable. They may form the earliest — 
stages of the change to the usual opaque earthy decomposing crust, 
in which, of course, all trace of any structure developed in the 
preliminary weathering is lost. 
In humid and temperate climates weathering is mainly due to 
the solvent influence of rain; in high mountainous situations, as 
well as in lower regions where the temperature falls below the ~— 
freezing point in winter, it is largely produced by the action of frost, 
to be afterwards described; im arid lands subject to great and rapid 
alterations of temperature it is caused by the strain of alternate 
expansion and contraction and the mechanical action of the wind 
(p. 319). As the name denotes, weathering is dependent on meteoro- 
logical conditions, and varies even in the same rock as these condi- 





























































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Fia. 84.—WEATHERED SANDSTONE CLIFFS SHOWING IRREGULAR HoNEYCOMBING AND 
WEATHERING ALONG PLANES OF STRATIFIOATION (B.). . 
tions change, but is likewise almost infinitely diversified according 
to the structure, texture, and composition of rocks, 
Mere hardness or softness forms no sure index to the comparative 
power of a rock to resist weathering. Many granites, for instance, 
weather to clay deep into their mass, while much softer limestones 
retain smooth hard surfaces. Nor is the depth of the weathered 
surface any better guide to the relative rapidity of waste. A 
tolerably pure limestone may weather with little or no crust, and 
yet may be continually losing an appreciable portion of its surface 
by solution, while an igneous rock like a dolerite or basalt may have 
a thick decomposed crust and yet weather with extreme slowness. 
In the former case, the substance of the rock being removed in 
solution, few or no insoluble portions are left to mark the progress 
of decay, while in the igneous rock the removal of but a com- 
paratively small proportion causes the disintegration of the rock, 
