BUILDING AND ORNAMENTAL STONES. 341 



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

 to be shown that rocks rich in triclinic feldspars 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 Portland, Boston, and Chicago not 

 merely exposed this delusion but proved the direct opposite — that in- 

 stead of being the most fire-proof 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 different degrees of expan- 

 sibility.! 



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 fire is stated 

 to have been well shown not long since at the burning of St. Peter's 

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

 great part of granite, was completely ruined, while the tower, built 

 of a local freestone, around which the beat of the fire 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 dolomites, 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 22, 1886, p. 75. 



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

 minerals lias been determined as follows : 



Tourmaline 000022 



Garnet 000025 



C ale i te 00002 



Dolomite 0000:i5 



Quartz 00003G 



Orthoclase 000017 



Adularia (feldspar) 0000179 



Hornblende 0000284 



Beryl 000001 



The quartz, it will be noticed, has a co-efficient of expansion double that of the 

 orthoclase, and nearly a third greater than hornblende. The matter is further com- 

 plicated by the fact that each individual mineral expands unequally along the direc- 

 tion of its various axes. Thus quartz gives a co-efficient of .00000709 parallel to the 

 major axis, and of . 00001385 perpendicular to this axis; adularia gives .0000156, 

 .000000059, and .00000294 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 sections of all our granites, such as are 

 used for building purposes, the author can but feel that in most cases the results 

 thus produced are too small to need serious consideration. 



$ Am. Arch., Vol. iv, 1878, p. 80. 



