378 BULLETIN OF THE NEW YORK STATE MUSEUM 



shaly cement, are Inherently weak. The ferruginous or Iron 

 compounds are all more or less easily attacked, and when, 

 as In the case of many sandstones, the cementing material 

 Is such. Its presence Is of Importance In a consideration as to 

 durability. The ferric oxide cannot take up more oxygen, 

 and Is, therefore, preferable to the ferrous compounds. 

 The sulphides are susceptible to change In the long ex- 

 posure to the air. The nature of the mineraloglcal species 

 In the case of the sulphides has, however, much to do with 

 the rate of decay, the pyrlte being more durable than the 

 marcaslte."^* 



Calcareous compounds, particularly the carbonates, are 

 dissolved readily by water, carrying carbonic acid gas, and 

 hence the pure limestones are comparatively short-lived. 

 The magneslan carbonate Is less soluble, and as a rule, the 

 maofneslan and dolomltic limestones and marbles are more 

 durable than the purely calcareous stones.f 



As there Is a wide range In the ease with which the more 

 common minerals of the crystalline rocks are attacked or In 

 their rate of alteration and decomposition, the same Is true 

 of the various granites, granitoid rocks, gneissic and other 

 crystalline stones In whose composition they enter, and 

 which are used In construction work. The feldspars vary 

 greatly In their ability to resist atmospheric agencies, which 

 tend to their kaolinlzatlon and destruction, as Is evident in 

 the unequal weathering of granitoid rocks under apparently 

 similar conditions. The more highly ferruginous micas, 

 hornblendes and pyroxenes are apt to decay more rapidly 

 than those containing less iron. Pyroxene, as compared 

 with hornblende, also is more liable to change. 



* A. A. Julien, United States Tenth Census, Report on Building Stone. 



f " As a general rule, however, the magnesian limestones, in their normal condition 

 are more friable and more porous and less firm in their character than the pure car- 

 bonates of lime. * * * The more porous limestones and some of the marbles, 

 which notoriously lack cohesive power, may be more affected by this action." Prof. 

 James Hall, report on building stone. 39th Annual Report New York State Museum, 

 Albany, 1886, p. 210. (Communicated to Capitol Commissioners in 1868). 



