THE AGE OF SOILS 376 



brilliant colors will be found to haye penetrated deepest along 

 joint lines, or the more porous portions, leaving the intervening 

 compact masses of more sombre hue. 



In discussing this matter, there is, however, one point that we 

 should not overlook, although its importance seems not to have 

 been fully realized by the authorities quoted, and that is, a 

 change in color due not alone to a change in the conditions of 

 the iron, but to the relatively greater abundance of this constitu- 

 ent in the uppei portions. The iron oxides, as already noted, 

 owing to their less soluble nature accumulate in the residues, 

 and as a rule, the more thorough the decomposition the greater 

 the proportional amount of iron. A small percentage of free 

 oxide disseminated throughout a relatively large amount of 

 detritus imparts but little color; the more iron, the more color. 

 The residue from the Medford diabase described on p. 200 is 

 of a deep brown color, as a whole, but the finest silt washed 

 from it is several shades brighter, of a dull ochreous red. Had 

 the entire mass decomposed to the condition of this silt, we 

 might expect it to have the same color. This change, due to 

 increased proportional amounts of iron oxides, is particularly 

 marked in limestone residuals where the original rock may con- 

 tain merely traces of free oxides, or ferruginous silicates. Neu- 

 mayer has shown^ that the snow-white Karst limestones contain 

 only some 0.044% of ferruginous silicates which themselves carry 

 20% of iron oxides. Yet the residual soil left by the decompo- 

 sition of this limestone is of so pronounced a color as to have 

 long ago received the name terra rossa or red earth. 



Other things being equal, brilliancy of color may then be 

 regarded as (1) indicative of advanced decomposition, and (2) 

 of geological antiquity. 



(7) The Age of Soils. — No sooner were the first rocks pushed 

 above sea-level than the various agencies described under the 

 head of weathering began the work of disintegration, decompo- 

 sition, and transportation. Of this we have ample proof in the 

 entire series of sedimentary rocks extending from the Archaean 

 down to the most recent and which are but the reconsolidated 

 residues of pre-existing masses. That such a breaking down 

 resulted in the production of soils is a fair inference, though 

 we have no absolute evidence of land plants and hence, a 

 priori, of soils, before the beginning of the Upper Silurian 



lYerliandl. k. k. Geol. Beichsanstalt, 1875-76, p. m. 



