548 A TREATISE ON METAMORPHISM. 



Surface induration, while most common as the result of the deposition 

 of silica, is not limited to this compound. The upward-moving- solutions 

 carried by molecular attraction may, under favorable circumstances, con- 

 tain iron salts in solution. When the solutions approach the surface evap- 

 oration takes place and the iron salts are thrown down, Usually as limonite 

 or hematite. If precipitated as limonite, this compound may be later 

 dehydrated and hematite be formed. Where the iron salts are transported 

 as carbonate, oxidation takes place at the time of precipitation. While it is 

 not necessary to suppose that the iron is always transported as carbonate, 

 since limonite and hematite are so insoluble it is natural to suppose that in 

 most cases the iron is transported in some other form than the oxide, and 

 is chemically changed when precipitated. By the above processes we have 

 surface induration due to iron oxide cement. This is beautifully illustrated 

 at various places in the desert regions of western United States. One of 

 the best localities known to me is that of the volcanic rocks of the Colorado 

 Desert east of Needles along the line of the Santa Fe Railway. The desert 

 is strewn with bowlders of disintegration, of a light-colored lava. The part 

 of these bowlders embedded in the sand has its normal color; the part 

 above the sand line is a rich reddish brown, due to hematite, the iron of 

 which has been brought to the surface in solution and has there been 

 oxidized and precipitated. 



Another interesting case of the segregation of iron oxide at the surface 

 is furnished by quartzite bowlders in the Potsdam sandstone of the driftless 

 area of the Baraboo district of Wisconsin. Here, locally, in consequence 

 of weathering processes, the sand matrix has been carried away and the 

 quartzite bowlders have accumulated at the surface. The quartzite is 

 ferruginous, containing hematite, which gives purplish and brownish .tones 

 to the unweathered rock. The centers of the bowlders still show these 

 colors; the borders are rich yellow-brown, or red, due to limonite and 

 hematite, the color being much more marked than in the centers of the 

 bowlders or in the massive quartzite, and plainly showing an unusual 

 proportion of the iron oxide. Between the outer surfaces of the bowlders 

 and the central cores of unaltered quartzite are frequently bands, from two 

 to five centimeters broad, almost white, which fade off into the heavily 

 ferruginous outer parts of the bowlders and to the less ferruginous cores. 

 It is very clear in these cases that the iron oxide once in this lighter middle 



