ON THE DURABILITY OF BUILDING STONE 377 



The coarsely-crystalline stones are more liable than the 

 finer-crystalline to be injured by the decay and falling out 

 of the less enduring micas and hornblendes. The disposi- 

 tion of the constituent crystals in any given stone also af- 

 fects its strength. The interlocking of crystals gives 

 strength, as in any woven texture the closer the threads of 

 woof and warp the greater the resistance to rending. 



II. Chemical Composition 



The durability of building stones is indicated by their 

 chemical composition, both in the cystalline and the non- 

 crystalline, or sedimentary groups. And the nature of both 

 the grains and the cementing material is to be considered. 

 The latter may be such as to be readily acted upon by 

 atmospheric elements, and the stone fall to pieces as a heap 

 of quartzose sand, each grain of which, by itself, would have 

 resisted for ages. Without the bond the tottering wall 

 gives away. The principal atmospheric agents which attack 

 stone, are carbonic, hydrochloric, nitric and sulphuric acids, 

 ammonia and several organic acids. These agents, carried 

 by rain water, act by solution, oxidation, deoxidation and 

 hydration and the constituent minerals as well as the cement 

 are affected by them. And the durability of any given 

 stone is determined by that chemical constitution which is 

 least liable to change under their action. 



In the crystalline rocks the varying degree of solubility 

 of the several minerals is indicative of the strength of resist- 

 ance, which may be offered, and of the enduring property of 

 the mass. A siliceous rock, other things being equal, is the 

 most durable. The silica must, however, be in a compact 

 and well cemented form, as in quartzite, and not in the shape 

 of a loosely coherent sandstone. Argillaceous or shaly 

 stones are inferior in quality on account of both physical 

 structure and chemical weakness. For the same reason 

 limestones containing clayey seams, and sandstones with 

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