THE COLOR OP ROCKS 43 



colors sometimes shown By labradorite-bearing rocks like those 

 of northern New York and of Norway is apparently caused by 

 a separation of the individual crystals along cleavage lines, into 

 thin, transparent plates which reflect and partially polarize 

 the light that would otherwise penetrate and become absorbed. 

 Through weathering, such feldspars undergo a further physical 

 change, becoming soft and porous, and no longer allowing the 

 light to penetrate, but wholly reflecting it, causing the stone to 

 appear white. These white feldspars, as has been very neatly 

 expressed by the late Dr. Hawes, bear the same relation to the 

 glassy forms as does the foam of the sea to the water itself, the 

 difference in color being in both cases due to the changed physical 

 condition. Indeed, the color of rocks, as may be imagined, is 

 not constant^ but liable to change under varying conditions. 

 Rocks black with carbonaceous matter will fade to almost white- 

 ness on prolonged exposure, owing to the bleaching out of the 

 coloring materials. Rocks rich in magnetite or free iron oxides, 

 protoxide carbonate, or sulphides, or in highly ferruginous 

 silicate minerals, are likewise liable to a change of color, be- 

 coming yellowish, red, or brown, through oxidation of the fer- 

 ruginous constituents. (See p. 243.) Translucent, nearly color- 

 less rocks or minerals, as those made up of crystals of calcite 

 or selenite, will on exposure become nearly opaque and snow- 

 white, owing to purely physical causes, as already noted in the 

 case of the feldspars. (See further in chapter on weathering.) 

 The cause of the color variations in certain rocks and min- 

 erals is, however, a matter concerning which it will not do, as 

 yet, to speak too decidedly. Analysis of a mineral may show 

 the presence of metallic oxides, but it does not necessarily fol- 

 low that whatever color the mineral may have is due to or in any 

 way related to these oxides. Thus the writer has shown^ that 

 the onyx marbles (travertines) of Arizona and Mexico may 

 vary from pure white to green, and from yellow through brown 

 to red, without appreciable change in the actual amounts of 

 iron though there may be a change in the form of combination. 

 In the white and green varieties the iron exists as a carbonate ; 

 in the yellow, red, and brown varieties as a more or less hydrated 

 sesquioxide. Certain dark amber and bright rose-colored va- 

 rieties from California, and the Californian Peninsula, show, 

 however, no iron or other of the usual metallic coloring con- 



^ Annual Report U. S. National Museum, 1893, p. 558. 



