522 REPORT OF NATIONAL MUSEUM, 1890. 



In still other cases, and particularly the feldspar- bearing* rocks, the 

 color may be dne in part to the physical condition of the feldspar. Thus, 

 in many rocks, like the norite, from Keeseville, New York (specimen No. 

 38744), the dark color is due in part to the fact that the feldspar is clear 

 and glassy, allowing the light rays to penetrate it and become absorbed. 

 When such rocks are exposed for a long period to the weather the feld- 

 spars frequently undergo a physical change, become soft and porous 

 and no longer absorb the light, but reflect it, giving the stone a white 

 color. 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. 



The color of rocks, as maybe imagined, is not constant, but liable to 

 change under varying conditions, particularly those of exposure. Eocks 

 black with carbonaceous matter will fade to almost whiteness on pro- 

 longed exposure, owing to the bleaching out of the coloring materials. 

 Eocks rich in magnetite or free iron oxides, protoxide carbonates, or 

 sulphides, or in highly ferruginous silicate minerals, are likewise liable 

 to a change of color, becoming yellowish, red, or brown, through oxida- 

 tion of the ferruginous constituents. 



In the series shown an endeavor has been made to arrange the rocks 

 in five groups, showing (L) rocks colored by carbonaceous matter; (2) 

 rocks colored by free oxides of iron ; (3) rocks colored by the preva- 

 lence of iron rich silicates ; (4) rocks, the color of which is due in part 

 at least to structural features, and the transparency of the feldspathic 

 constituent; and (5) rocks, the color of which is also in part due to the 

 physical condition of the various constituents, but more particularly 

 to a lack of carbonaceous matter, iron, or other metallic oxides. To 

 this series is appended another, showing the changes in color due (1) to 

 the bleaching of the carbonaceous matter ; (2) to the leaching out of 

 the ferruginous oxides by organic acids ; (3) to the oxidation of iron pro- 

 toxide carbonates or sulphides '; (4) to a like change in the iron rich sili- 

 cates ; and (5) to a change in the physical condition of the constituent 

 minerals, mainly the feldspars. 



I. Colors due to carbonaceous matter: 



(1) Black. Black marble, Glens Falls, New York. 26163 



(2) Dark gray. Limestone, Schoharie County, New York. 25909 



(3) Blue. Limestone, Murphy, North Carolina. 9-765C 



(4) Dark gray. Carbonaceous shale, Tsere, France. 38176 



(5) Dark gray. Slate, Savoy. 38194 



II. Colors due to free iron oxides : 



(1) Dull brown. Sandstone, Washington County, Kansas. 56963 



(2) Reddish brown. Sandstone, Rusk, Texas. 35574 



(3) Red. Sandstone, Seneca Creek, Maryland. 69283 



(4) Red. Quartzite, Rock County, Minnesota. 37407 



(5) Light red. Volcanic Tuff, Nevada. 35381 



(6) Yellow. Limestone, Wilson, Kansas. 26482 



