CHAPTER XV 



BRICK AND BUILDING STONE 



221. Limestone. Limestone is principally a carbonate of lime, made 

 up of seashells that have been deposited from water during past 

 geological times. Its method of formation has much to do with its 

 value as a building material. If it contains no thin layers of clay 

 or shale (sedimentary planes), it is likely to be fairly homogeneous in 

 structure. But if layers of shale, however small, occur, the material 

 is much more quickly weathered. This is especially true if the stone 

 be placed at right angles to the position it occupied in the quarry. 



Thin planes of foreign substances are likely to occur in many 

 of our best building stones, as may be seen in the rapid deteriora- 

 tion of seemingly first-class limestone when used as curbing. Such 

 disintegration is caused by a lessening of the adhesion between the 

 particles of stone. 



Limestone may be composed of a great percentage .f >and c.'mented 

 together by calcareous matter, in which ease it i< railed siliceous /////<- 

 stone. Under such eireum-tances chemical action may remove tin- 

 cementing material, thus leaving the stone free to crumble. Marble 

 is almost pure limestone. 



Conditions to which a building stone is to be exposed will determine 

 the character of the material to be used in any particular structure. 

 Rapid freezing and thawing is likely to set up internal strains, in the 

 material, which may lead to future failure. These strains may be 

 caused by unequal expansion or contraction of the particles of the 

 stone, or by the freezing and thawing of the water in the stone. The 

 formation of ice in the sedimentary planes accounts in a large measure 

 for the rapid deterioration of stone. 



Limestone often occurs in very thick layers, a^ in the case of the 

 oolitic limestone found at Bedford, Indiana, where the layers are 

 often from 25 to 30 ft. thick. In such cases it is a most valuable 



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