ON THE DURABILITY OF BUILDING STONE 379 



The mineralogical composition representing the chemical 

 nature of the stone is, therefore, of the first importance in a 

 consideration of its durability for building purposes. And 

 for the granites a safe generalization is a relatively high per- 

 centage of silica, as quartz (or an acidic granite) with un- 

 altered, orthoclase feldspar and a comparatively small 

 amount of the lighter-colored hornblendes and micas. In 

 the marbles the dolomitic and highly-crystalline varities are 

 to be preferred to the purely calcareous stones.* In the 

 limestones the more siliceous varieties and those in which 

 the magnesia and lime are found in the proportions of a true 

 dolomite, represent the more durable kinds. The sandstones 

 vary much in the nature of the cementing material, and they 

 are graded, according to their chemical composition, into 

 siliceous, argillaceous, ferruginous, micaceous and calcareous 

 varieties. The siliceous bond is the best, wherein the mass 

 approximates to a quartzite in its composition. The others 

 are to a greater or less degree liable to decay through the 

 solution of their cement. 



III. Accident of position — in use 



In this section the consideration of the properties inherent 

 in stone, due to their position in their natural habitat — the 

 quarry, is omitted, although its importance is recognized. 

 We take it from Nature's great building for the construction 

 of our edifices. Reference, however, may be made to the 

 weathered rocks of the surface, the altered laminae of beds 

 standing on edge, the shattered condition of highly folded 

 and faulted strata, the metamorphosed nature of the rocks 

 of volcanic districts, and the glaciated and polished out-crops 

 within the limits of the continental glacier. In these cases 

 Nature is a great teacher, and her object lesson, so easily 



* The rapid wear and destruction of some well-known marbles is explained by the 

 mixture of calcareous with dolomitic grains and the disintegration, by the solution of 

 the former — thus breaking down the mass. 



