244 THE PHYSICAL MANIFESTATIONS OP WEATHEEING 



the ^^dikcvmm gres/' oxidized and impoverished in lime by the 

 action of meteoric waters. 



The same feature is noticeable in many of our quarries for 

 building stone, as those m the Berea sandstones of Ohio. The 

 beds below the drainage level, are of a gray or blue-gray color, 

 while above, where they have been subjected to the oxidizing 

 influence of meteoric waters, they are buff. The Jurassic oolites 

 of England, are blue-gray at some depths below the surface, but 

 white above. 



In cases where natural joint blocks are exposed to the perco- 

 lation of meteoric waters, the weathering may for a time mani- 

 fest itself only in differential oxidation and zonal segregation 

 of the iron whereby are produced concentric bands of varying 

 hues. Fig. 3, PL 22, is a slab from a natural joint block of 

 argillite in the collections of the National Museum, in which 

 the bands, due to this cause, vary from yellow-brown, drab, to 

 ochreous yellow and red, while the rock as a whole still retains 

 its compact structure and susceptibility to polish, forming an 

 ornamental stone of no mean order.^ 



W. P. Blake has described boulders from the Colorado desert 

 colored exteriorly by what he regarded as organic matter re- 

 ceived from water during a period of submergence. Similarly 

 discolored quartzite boulders brought by G. K. Gilbert from the 

 Sevier desert in Utah, and examined by the present writer, show 

 a thin dark varnish-like coating, not inaptly named by Mr. 

 Gilbert '^desert varnish," and which consists largely of oxides 

 of iron and manganese, though a slight amount of organic 

 matter is present. In this case the rock is composed not wholly 

 of quartz granules, but carries interstitial calcite and feldspathic 

 granules. Near the discolored surface of the boulders these in- 

 terstitial ealcites are found quite dissolved away, leaving cavities 

 stained by a dark deposit which reacts for iron and manganese. 

 Inasmuch as acid solutions obtained from fresh and uncolored 

 portions of the boulders give faint reactions of the same nature, 

 it seems very probable that the crust is due to a concentration 

 of these metals in a condition of higher oxidation on the surface, 

 whither they have been brought by capillarity, while the more 

 soluble lime carbonate was removed. ^ It is freely acknowledged, 



J stones for Building and Decoration, p. 169. 



2Althoug]a such discolorations seem to have been noted principaUy in 

 desert regions, they are by no means limited thereto. The quartzitie boulders 



