BUILDING AND ORNAMENTAL STONES. 345 



Acid gases are naturally without effect upon the silicious particles 

 of a sandstone, and can be productive of injury only in dissolving out 

 the ferruginous and calcareous cements. This is actually accomplished 

 in many cases, and much disintegration results as a consequence. In- 

 deed, Egleston* seems to regard the serious decay into which the stone 

 of Trinity Church, New York, has fallen, to be due chiefly to this cause, 

 supplemented by the action of frost after the cement had been removed 

 and the stone thus rendered porous. The relative solubility of the vari- 

 ous ferruginous cements has been already alluded to (ante p. 339). Oxi- 

 dation is likely to play a more noticeable part in sandstones than in most 

 other rocks, owing to their porous nature, which allows ready access of 

 water and air. The effect of oxidizing pyrite in producing the mellow- 

 ing and other color changes in stones of this class is sufficiently dwelt 

 upon elsewhere, as is also the effect of heat, both natural and artificial. 



On account of their porosity and natural roughness of surface sand- 

 stones are of all stones most likely to afford foothold for the growth of 

 alga3, lichens, and mosses. While it is yet to be proven that these are 

 actually injurious, they are at least suggestive of an unhealthy damp- 

 ness. A stone once covered by these organisms will absorb more water 

 and give it up more slowly to evaporation than one whose surfaces are 

 not thus protected. 



Serpentines when free from bad veins are as a rule non-absorptive 

 and not affected by gaseous atmospheres, hence are durable if free from 

 bad joints. The Pennsylvania serpentines sometimes turn whitish on 

 exposure, but so far as observed do not disintegrate. 



Soapstone, although too soft and possibly too slippery for general 

 building, is nevertheless one of the most durable stones, being not only 

 proof against atmospheric and chemical agencies, but when well sea- 

 soned fire-proof as well. 



Gypsum is too soft and too soluble in ordinary terrestrial waters to 

 be of great value. 



I.__ON THE SELECTION OF BUILDING STONE. 



(1) GENERAL CONSIDERATIONS. 



From what has gone before it must be evident that there are many 

 more factors which go to determine the value of stone for structural 

 purposes than are ordinarily taken into consideration. It may there- 

 fore not be out of place here to mention a few general principles to be 

 observed in selecting stone for any purpose in which durability or 

 stability of color are matters of importance. It should be stated at the 



doubt, get saturated. This means expending more fuel to convert part of this water 

 into vapor. The difficulty is surmounted to «'i great extent by building hollow walls, 

 the inner wall being of brick. Woe unto the man who has not t:ikYn this precaution." 

 (T.Mellard Reado, in Proe. Liverpool Geological Soc, p. 445 and 440, 1883-84.) 



"Cause and Prevention of Decay in Building Stone. Trans. Am. Soc. Civ. Engs, xv, 

 1886. 



