CASEHARDENING AND DESERT VARNISH. 549 



band has been taken into solution, transferred to the surface, and there 

 deposited. It is a depleted area, and correlative with it is the surplus of 

 iron oxide at and near the surface. This case is interesting, since the iron, 

 both in its original form and where segregated, is an oxide. It is therefore 

 clearly shown that iron in the form of hematite is soluble in the belt of 

 weathering. Whether actually carried in that form or whether reduced 

 and changed to carbonates or other salts at the time of transfer is uncertain. 



In a similar manner other cements besides silica and iron oxide may 

 be brought to the surface and deposited. These processes, while most 

 prominent at the surface, are not limited to it; they take place to a consid- 

 erable extent along the major openings of the rocks near the surface, such 

 as joints. Crosby has noted the induration of Pikes Peak granite along 

 the joints. Surface induration of granite, rhyolite, and other rocks has 

 been noted, the casehardening materials ordinarily being the three most 

 common ones — silica, iron oxide, and calcite. a 



In connection with casehardening it is interesting to note the influence 

 of the so-called quarry water contained in stones taken from the quarry. 

 This is the water of imbibition. As this water is evaporated the material 

 in solution is deposited between the grains near the surface and thus helps 

 to cement them. The very considerable induration thus produced by the 

 quarry water is evidence that this water is rich in mineral solutions. In 

 this deposition of material we have the explanation of the great advantage 

 of dressing sandstones and other porous rocks before the quarry water is 

 lost by evaporation. 



From the previous pages it appears that capillarity is most effective in 

 transferring soluble material from below the surface to the surface in semi- 

 arid and arid regions. But if in regions which are ordinarily known as 

 humid there are seasons of drought, great quantities of soluble material 

 may be segregated at the surface by capillarity. So far as this material is 

 thrown down in a readily soluble form it is likely to be largely taken into 

 solution at times of abundant rainfall and again carried below the surface. 

 But if at the time of precipitation chemical change takes place, so that the 

 material is transformed to a relatively insoluble form, as, for instance, the 

 delwdration of colloidal silicic acid forming quartz, or the oxidation of iron 



"Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, pp. 

 254-256. 



