BUILDING AND OKNAMENTAL STONES. 337 



Chemical action of the atmosphere. — The series of changes induced by 

 these agencies are, as above indicated, chemical in their nature and may 

 all, as first suggested be conveniently grouped under the heads of 

 oxidation, deoxidation, hydration, and solution. These may as well 

 be considered in the order given. 



Oxidation. — The process of oxidation is commonly confined to those 

 stones which carry some form of iron as one of their constituent parts. 

 If the iron exists as a sulphide (pyrite or marcasite), it very probably 

 combines with the oxygen of the air on exposure, forming the various 

 oxides of iron such as are popularly known as " rust." If the sulphide 

 occurs scattered in small particles throughout a sandstone the oxide 

 is disseminated more evenly through the mass of the rock, and aside 

 from a slight yellowing or mellowing of the color, as in certain of the 

 Ohio sandstones, it does no harm. Indeed, as suggested by Professor 

 Winchell, * it may result in positive good, by supplying a cement to the 

 individual grains, and thus increasing the tenacity of the stone. In all 

 other than sandstones, however, the x>resence of a readily oxidizable sul- 

 phide is a serious defect, since crystalline rocks require no such cement, 

 and the change in color can in very few cases be considered other than 

 a blemish. This is well illustrated in some of the lower courses of 

 granite in the new capitol building at Albany, New York, to which 

 reference has already been made. More than this, the pyrite, in decom- 

 posing in contact with the gaseous atmosphere of cities, may give rise to 

 small quantities of sulphurous and sulphuric acids, which by their cor- 

 rosive action upon the various mineral constituents of the stone render it 

 porous and more liable to the destructive effects of frost. (See p. 301.) 

 The conversion by oxidation of a sulphide into a sulphate is moreover 

 attended with an increase in volume ; there is thus brought to bear a 

 mechanical agency to aid in the work of disintegration. 



Iron in the form of a ferrous carbonate is a common constituent of 

 many calcareous rocks, and in the form of other readily decomposable 

 protoxide compounds occurs not infrequently in the cementing material 

 of fragmental rocks lying below tlie water level. All these compounds 

 are susceptible to oxidation on exposure to atmospheric influences, and 

 to these, more than to the presence of sulphides is presumably due the 

 mellowing commonly observed in white marble or the light gray sub- 

 Carboniferous sandstones. 



in the atmosphere entirely in an unconibihed state, but were probably in largo part 

 combined with otber substances to form chlorides, sulphates, etc. L. P. Grata- 

 cap (School of Mines Quarterly, May, 1885, p. 335), from a series of tests at Staten 

 Island, New York, computed the entire amount of chlorine brought down by the 

 rains during 1884 to have been some 46.23 pounds for each acre of ground. This is 

 regarded as in large part combined with sodium to form sodium chloride (common 

 salt). Egleston (Cause and Decay of Building Stone, p. 5) estimates that the 4,500,000 

 tons of coal annually burnt in New York City discharge into the air 78,750 tons of 

 sulphuric acid. In (>5 cubic centimeters of rain-water caught during an exposure of 

 forty-one days, this same authority found 4 J- milligrams of sulphuric acid. 

 * Geol. of Minn., Vol. I, p. 189. 



H. Mis. 170, pt. 2 82 



