288 STUDIES FOR STUDENTS 



a precipitate in the laboratory. But this colloid mass, even if immersed in water 

 and entirely undisturbed, gradually and spontaneously gives off a large part of 

 the water which the ferric oxide has so greedily absorbed vs?hen in the nascent 

 state; and it appears thus, as it slowly solidifies and hardens, to pass in succes- 

 sion through the forms of the various native yellow hydrates — limnite, xantho- 

 siderite, and limonite, to gothite. That this progressive change continues is 

 evident from the fact that these yellow hydrates are gradually replaced in the 

 older formations by the red hydrate (turgite) and by ferric anhydride (hematite). 

 When occurring as original or contemporaneous, and not as secondary, deposits, 

 the yellow ores of iron are found, as a rule, only in the later rocks; while the red 

 ores are generally restricted to the earlier rocks. This genetic relation of the 

 yellow and red ores is one of the most familiar and generally accepted facts in 

 geology. However recent the origin of the red ore (turgite or hematite) may 

 appear to be in any case, we naturally infer that it was first yellow, and that it 

 has passed slowly or rapidly, as the case may be, but gradually, through the 



series of yellow hydrates 



If it be conceded that the dehydration is virtually, if not absolutely, spon- 

 taneous, and there is no apparent alternative, it follows that the color of a deposit, 

 so far as it is due to ferric oxide, is, other things being equal, a function of its 

 geological age. In other words, the color naturally tends with the lapse of time 

 to change from yellow to red; and, although this tendency exists independently 

 of the temperature, it is undoubtedly greatly favored by a warm climate. Apply- 

 ing this principle to the sedimentary soil of the South, we find that the super- 

 ficial portion is red, not alone because it is exposed to a higher temperature than 

 the subjacent yellow clay, but also because it is the oldest part. On the other 

 hand, the limited occurrences of post-glacial sedimentary detritus in the North 

 are, in the absence of the favoring climatic influence, still too young to exhibit 

 the change of color even superficially.' 



Judging from the later expressions of opinion, this article does not 

 seem to have received the attention which it deserves. 



Spontaneous dehydration assisted by heat and favored by time does 

 not appear, however, to be the sole cause of the great contrast in color 

 between the consolidated and the surficial ferruginous sediments, a 

 still more potent cause existing in the dehydration effected by the 

 great increase in pressure and moderate rise in temperature which 

 takes place upon the burial of the material to some thousands of feet 

 beneath later accumulations. The efficiency of pressure in this con- 

 nection is exhibited in the formation of shales, where about one-haK 

 of the combined water is eliminated at temperatures which must be 

 frequently far below the boUing-point, since with the normal gradient 



I Op. cit., pp. 80, 81. 



