BUILDING AND ORNAMENTAL STONES. 341 



crumbling.* This view seems phxnsible, but we believe it yet remains 

 to be sbown that rocks rich in triclinie fel(lsi)ars in reality disintegrate 

 more rapidly than others. 



Granite was for a long time popularly believed to be a nearly fire- 

 proof material. The great fires of Tortland, Boston, and Chicago not 

 merely exjiosed this delusion but proved the direct opposite— tliat in- 

 stead of being the most tire-jiroof it was the least so, ranking below 

 either sand or limestone. The peculiar susceptibility of the stone to 

 the effect of heat may be ascribed to its (compact and complex structure, 

 each of its constituent minerals possessing diiferent degrees of expau- 

 sibility.t 



It has also been suggested by certain authors that the minute water- 

 filled cavities in the quartz of these rocks may be an important factor, 

 since, when highly heated, the water is converted into steam and an 

 explosion results, causing the quartz to lly into fragments. | 



The relative durability of sandstones and granite under lire is stated 

 to have been well shown not long sin(;e at the burning of St. Peter's 

 Church at Lamerton, England. The church itself, which Avas built in 

 great part of granite, was completeiy mined, while the tower, built 

 of a local freestone, around which the heat of the tire was so great as 

 to melt six of the bells as they hung in the belfry, was left intact, 

 although the granite window-jams and sills were destroyed.§ 



Limestones and doloinites, both marbles and the common varieties, are 

 perhaps less affected than granite by the purely mechanical agencies, 

 but make up for this in their susceptibility to the solvent action of gas- 

 eous atmospheres. Limestones are in this respect less durable than 

 dolomites, so that, the tenacity being the same, a dolomite might, under 

 the same circumstances, be considered as promising greater durability 



* See Science, January "^, 188(), p. 75. 



t The co-efficient of cubical expansion for several of tlie more common rock-forming 

 minerals has been determined as follows : 



Tourmaline 000022 



(iarnet 000025 



Calcite 00002 



Dolomite 0000:!5 



Quartz 0000:U) 



Orthoclase 000017 



Adularia (feldspar) 0000179 



Hornhlcnde 0000281 



Beryl 000001 



The (piartz, it will be noticed, has a co-cllic.icnt of expansion double that of (ho 

 orthoclase, ami nearly a third <^rcater than hornblende. The nnitter is further com- 

 plicated by the fact that each individual nuneral (>,x])ands unecjually alon<^ the direc- 

 tion of its various .axes. Thus quartz gives aco-ef(ici(!nt of .(K)0007fi9 parallel to tln^ 

 nijijor axis, and of .00001 ;)85 perpendicular to this axis; adularia gives .000015(), 

 .00()000(;51), and .00000291 for its three axes ; and hornblende for the same axes gives 

 .0000081, .00000084, and .0000095. (See Clarke's Constants of Nature, Smithsonian 

 Misc. Coll., Vol. XIV.) 



t After a microscopic examination of thin seditions of all our granites, such as are 

 used for building purposes, the author can but (eel th;it in most cases the results 

 thus produced are too small to need sei-ious (sonsidera-tion. 



§ Am. Arch., Vol.iv, 1878, p. 80. 



