QUARRY MATERIALS OF NEW YORK 3 1 



the two together in sedimentary rocks. Iron occurs in chemical 

 combination chiefly in the silicate minerals like biotite, hornblende, 

 augite and olivine, lending various shades of green or a black color 

 to these ingredients of the crystalline rocks. It also occurs in the 

 form of free oxides, sulphides and carbonate distributed through 

 the body of the rock. The yellow, brown and reddish tints are 

 mainly due to the oxides of iron, blue and gray to the carbonate. 

 Carbon occurs in finely divided particles which lend a black or 

 bluish color to certain limestones, marbles and slates. 



The presence of iron in a condition of incomplete oxidation, as 

 ferrous oxide or carbonate, or as a sulphide, is detrimental to build- 

 ing or ornamental stones. The original colors incident to their 

 presence will not prove permanent. In some classes of material, 

 ,the change which takes place by oxidation of these compounds 

 produces a desirable mellowing effect, as in the Hudson River 

 sandstones, but ordinarily it leads to red or yellow blotches. The 

 colors resulting from the oxidation of pyrite and marcasite are 

 also apt to run, forming streaks which extend outward from the 

 particles and are quite frequently seen in exposed walls. Some 

 measure of the permanancy of color in building materials may be 

 had by a chemical analysis giving the percentages of unoxidized 

 iron. Allowance should be made for the nature of the compound, 

 for the mineral magnetite which contains both ferrous and ferric 

 iron is more stable under atmospheric weathering than a ferrous 

 compound like the carbonate. In fact magnetite is extremely re- 

 sistant to change and its occurrence can not be held as a draw- 

 back to the use of any stone. 



Besides the change of color that takes place in building stones 

 through the relatively slow alteration of the components as noted, 

 there are well-known instances where changes occur almost im- 

 mediately on removal of the stone from the quarry. The nature of 

 this change is not fully understood, but it seems to be connected 

 in some cases with the loss of the quarry moisture or sap. As a 

 local example may be cited some of the occurrences of the Adiron- 

 dack green syenite which have a lively light to dark green color 

 on fresh surfaces but which change within a few days to a yellow 

 or muddy green. The change is unaccompanied by any discernible 

 effect with respect to the mineral ingredients, and, though it seems 

 to be connected with the loss of moisture, the original tint can not 

 be restored by long-continued immersion in water. 



The appearance of stone in a building can not be summed up 

 entirely under color. Some kinds have a bright, clean look which 



