CLIMATE AND TERRESTRIAL DEPOSITS 289 



a temperature of 110° C. is attained only at a depth of about 11,000 

 feet (3,300 meters). The ferric oxide, holding its water with much 

 less tenacity than the silicate of alumina, seems to respond most readily 

 to the influence of pressure, giving rise to minerals of notably less 

 volume and greater density, apart from the water which is eliminated 

 in the process. 



Still a third factor in the development of a red color in ferruginous 

 rocks is found in the physical state of the oxide, as may be seen upon 

 contrasting the brilliant color of earthy hematite with the deep colors 

 of the same mineral in its crystalline form. The red color depends 

 therefore not only upon the presence of anhydrous or partially anhy- 

 drous ferric oxide, but also upon a fine state of division and diffusion. 

 Dawson speaks of the very fine state of division of the red coloring 

 matter in the lower Carboniferous of Nova Scotia — 



having indeed the aspect of a chemical precipitate rather than of a substance 

 triturated mechanically. In addition to the oxide of iron distributed through 

 the beds, there is, in the fissures traversing them, a considerable quantity of the 

 same substance in the state of brown hematite and red ochre, as if the coloring 

 matter had been superabundant or had been m part removed and accumulated 

 in these veins. ^ 



Hilgard states further that the general red aspect of tropical soils 

 is by no means always accompanied by markedly high percentages of 

 ferric oxide, but the latter is very finely diffused so as to be very effect- 

 ive in coloration. The soils of the arid regions on the other hand 

 are not deficient in ferric oxide. ^ The present writer has also noted 

 that pebbles of feldspar showing glistening cleavage embedded in 

 the red Triassic arkoses of the eastern United States are stained red 

 throughout with ferric oxide while those of quartzite are stained to 

 variable depths and those of vein quartz only along the fractures. 

 The completeness of the staining, its development about the stream- 

 worn surfaces of pebbles, and its presence in all materials, depending 

 only upon their porosity, are indications that this was done after incor- 

 poration in the sediments and through a considerable period of time. 

 The above facts seem to show that ferric oxide in rocks is rather readily 



1 J. W. Dawson, " On the Coloring Matter of Red Sandstones and of Greyish 

 and White Beds Associated with Them," Quarterly Journal 0} the Geological Society , 

 Vol. V, 1848, pp. 25, 26. 



2 Hilgard, Soils, 1906, pp. 392, 400. 



