RED COLOR OF THE SOIL. 161 



cated by the ignition, has increased very greatly, as was to be expected. 

 The small original amount of phosphoric acid prohibits our placing too 

 much reliance upon an indicated gain of the fraction of one per cent in 

 this constituent, since it may be due to errors in manipulation. 



On the assumption that such calculations always yield results rather 

 under than above the amount of material actually lost, I have assumed 

 from this and other analyses available that a silicious crystalline rock of 

 the granitic or more basic type in passing into the condition of a residual 

 clay or soil, as the word is ordinarily used, loses about 50 per cent of its 

 original constituents through solution and the leaching action of meteoric 

 waters.* 



Cause of Red Color of the Soil. 



The pronounced red color of this residuary material is worthy of con- 

 sideration. It will be remembered that the cause of this striking differ- 

 ence in the color of superficial deposits in northern and southern latitudes 

 has been the subject of much discussion, the names of Professors W. 0. 

 Crosby ,f I. C. Russell, X and J. D. Dana§ being most conspicuous in ex- 

 isting literature. Professor Crosby argued that the intense color was 

 purely superficial and due to a dehydration of the ferric salts with which 

 the soils are impregnated, whereby the color was changed from yellow 

 and brown to red, the cause of the hydration being the long continued 

 higher temperatures to which the superficial portions were subjected in 

 the lower latitudes. It is not necessary to go into a detailed discussion 

 here of all the points involved in the controversy, but I wish to state that 

 my own observations are entirely in accord with those of Professor Crosby, 

 so far as the superficial character of the phenomenon is concerned. In 

 some cases the brilliant color is found as a mere wash, or again it pene- 

 trates to the depth of a foot or more before giving wa}^ to more modest 

 hues. In such instances the colors have penetrated most deeply along 

 joint planes or other lines of weakness, leaving the intervening compact 

 masses of more somber shade. The gradual transition from the surface 

 downward, from bright red to brown, yellow, or gray, may be seen to 

 have taken place, usually within the space of a few vertical feet, in any 

 new road or railway cut, but becomes quickly obscured, even by the first 

 rainfall, through the washing down of the ochreous coloring matter from 

 above. 



That the increased color in the superficial portions is due wholly to 

 dehydration of the ferric salts, the writer is disposed to doubt. It must 



*See Rocks, Rockweathering, and Soils. The Macmillan Company. New York, 1897. 



t Proc. Boston Soc. of Nat. Hist., vol. 23, 1875, and Am. Geologist, August, 1891. 



t Bull. U. S. Geol. Survey, no. 52, 1889. 



g Am. Jour, of Science, vol. 39, 1890, pp. 317-319. 



