CASEHARDENING AND DESERT VARNISH. 547 



thin outer layer or crust of quartzite. Sections of such material show that 

 it is a true cementation quartzite. (See pp. 865-868.) Casehardening also 

 occurs in many other kinds of rocks, for instance, granite. In the produc- 

 tion of casehardening the waters have penetrated to a greater or less depth, 

 and through capillarity have been brought back to the surface as a result 

 of drying of the outer surface. 



During the journey of the water silica is dissolved, probably mainly in 

 the colloidal form. This H 4 Si0 4 is doubtless largely that liberated from 

 the silicates during the process of carbonation. (See pp. 473-480.) Prob- 

 ably the solutions do dissolve some silica from chert and quartz and bring 

 it to the surface, but the amount of this is probably small as compared with 

 that transported in the colloidal form. It therefore appears that the process 

 of surface cementation, so far as the cement is silica, is a correlative of the 

 process of carbonation. 



Very near the outer surface of the rock the water evaporates, the silica 

 is dehydrated, and quartz is deposited. In this case chemical change takes 

 place. The silica is thrown down in a very difficultly soluble form; so that 

 even in humid regions the falling water is unable to dissolve the quartz and 

 carry it downward. This process may continue until the sandstone, which 

 may be loose or almost incoherent below the surface, is a quartzite at the 

 surface. The casehardened layer may be a mere outer film but a fraction 

 of a centimeter in thickness, perhaps one-half to one-fourth of a centimeter. 

 In other instances the casehardening may extend to a depth of a centi- 

 meter, or even several centimeters. Ordinarily the maximum induration is 

 at the surface, and the transitions between the indurated and soft parts of 

 the rocks are somewhat rapid. This fact gives positive evidence that 

 evaporation and consequent deposition are at a maximum at the surface. 



Casehardening has been observed in many parts of the world. It is 

 beautifully illustrated at many points in the arid region of western United 

 States'. In arid regions the hardened film has frequently been smoothed by 

 the wind-blown sand, so as to present a polished surface. Such polished 

 hardened films are known as "desert varnish." While more common in 

 arid regions, casehardening has been found in humid regions. For instance, 

 the phenomena are well exhibited by the Potsdam and St. Peter sandstones 

 of Wisconsin." 



<"' Wadsworth, M. E., Some instances of atmospheric action on sandstone: Proc. Boston Soc. 

 Nat. Hist., vol. 22, 1883, p. 202. 



