BUILDING AND ORNAMENTAL STONES. 301 



ingredient whose presence can result in little that is good and perhaps 

 a great deal that is bad. It should be noted that pyrite on decompos- 

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

 which in themselves aid in the work of disintegration. 



" 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 efflorescent salt, which 

 exudes to the surface and forms white patches, which are alternately 

 washed oft' and replaced, but leaving a whitened surface probably 

 from the presence of sulphate 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 uufrequently observed in buildings constructed of the 

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

 the efflorescent patches proceeding from some of the joints between 

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



MAGNETITE. Magnetic Iron Ore. — Composition : FeO-fFejOa = iron sesquioxide, 

 G8.97 per cent. ; iron protoxide, 31.03 per cent. Hardness, 5.5 to C.5. 



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

 in the latter usually in minute crystals visible only with the microscope. 

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

 and basalt. When present in considerable quantities it sometimes 

 becomes converted entirely into the sesquioxide of iron through taking 

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

 color, as is observable in many diabases. 



HEMATITE. Specular Iron Ore. — Chemical composition : Anhydrous sesquioxide 

 of iron, Fe 2 3 ,= iron, 70.9 per cent. ; oxygen, 30.20 per cent. 



This mineral occurs in varying proportions 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 turgitesmd limonite, which are the forms occurring 

 in the Triassic saudstones of the eastern United States.f 



* Hall. Report on Building Stone, p. 50. The white efflorescence go frequently 

 seen on stone and brick buildings, seems, according to good authorities, to bo, in 

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

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

 present work. 

 " + Julien, Proc A. A. A. S., 1878. 



