BUILDING AND ORNAMENTAL STONES. 301 



iDgredient whose presence can result iu little that is j^ood and perhaps 

 a great deal that is bad. It should be noted that i)yrite on decompos- 

 iug, may give rise to sulphates and perhaps to free sulphuric acid, 

 which in themselves aid in the work of disiutegratiou. 



"In limestones or dolomites the presence of iron pyrites operates dis- 

 astrously; for, if magnesia be present, the sulphuric acid from the 

 decomposing iron pyrites produces a soluble elllorescent salt, which 

 exudes to the surface and forms white i)atches, which are alternately 

 washed off and replaced, but leaving a whitened surface probably 

 from the presence of suljdiate of lime. If the limestone be entirely 

 calcareous, the salt formed (a sulphate of lime) is insoluble, and 

 therefore produces less obvious results. In some cases, however, the 

 lime of which the mortar or cement is made may contain magnesia, 

 and the decomposition of the iron pyrites in the adjacent stone pro- 

 duces an efflorescent salt which exudes from the joints. This con- 

 dition is not unfrequently observed in buildings constructed of tlie 

 bluestone of the Hudson Eiver group. As an example, we may notice 

 the efflorescent pat(;hes proceeding from some of the joints between 

 the stones of St. Peter's Church, on State street, in Albany."* 



MAGNETITE. Magnetic Iron Ore. — Composiiio7i : FeO+FeiOa = iron sesqiiioxide, 

 (l.~^.'.)7 pt'iTciit. ; iron protoxide, ril.O'J per cent. Hardness, 5..^ to 0.5. 



This occurs as an original constituent in many schists and granites; 

 in the latter usually in niinute crystals visible only with tbe microscope. 

 It is almost invariably present in igneous rocks such as diorite, diabase, 

 and basalt. When present in considerable (juautities it sometimes 

 becomes converted entirely into the sesquioxideof iron through taking 

 oxygen Irom the the atmosphere. It then stains the rock a rusty red 

 color, as is observable in many diabases. 



HEMATITE. Specular Iron Ore. — Chemical compoisiiion : Anliydrous sesqnioxide 

 of iron, FeiOa,^ iron, 70.9 per cent. ; oxygen, 30.20 per cent. 



This mineral occurs in varying jiroportions in rocks of all ages. In 

 granite it usually occurs as minute scales of a blood-red color. In the 

 amorphous form it often forms the cementing material of sandstones, 

 when it imparts to them a red or reddish-brown color. This form of iron 

 oxide is, however, less common as a cementing substance than the 

 hydrous sesquioxides turgiteiiwCi Unionite, which are theforn)s occurring 

 in the Triassic sandstones of the eastern United States.! 



* Hall: Report on Building Stone, p. 50. Tbo white elllorescence eo frequently 

 seen on stone and brick buildings, seems, according to good antboritics, to be, in 

 most cases, due to tbe mortar in which the stone is laid, and is not an inherent qual- 

 ity of tlie stone itself. The subject is, therefore, not more fully dwelt upon in the 

 present work. 



t Julien, Proc A. A. A. S., 1878. 



