214 [Assembly 



sustained. In a large and lieavy building it is all important that the 

 foundations be firm and unyielding, for on this depends the integrity 

 of the entire structure. Beyond this it is important that the stone 

 be evenly cut, so that the bed of each succeeding block should rest 

 evenly upon those below it. From an inequality in dressinff two 

 adjacent blocks of stone to the same thickness, leaving at their junc- 

 tion one of them projecting slightly beyond the other, I have seen tlie 

 superincumbent block of granite cracked quite through. This break- 

 ing was not due to pressure alone, nor to want of strength in the 

 material, as was evident from the perfection of the wall below, but 

 entirely to the pressure bearing upon the center of a block resting on 

 an uueveu bed, or supported at the two ends and not in the center. 



VUl. 



Causes affecting the Durability of a Building Stone, which 

 ARE Inherent in the Stone Itself. 



The causes of disintegration and destruction in the ordinary build- 

 ing stones have already been mentioned under each one. They may 

 be recapitulated, however, in this place. 



1. Want of proper cohesion among the pariides proilncing inherent 

 weakness. This condition may arise from the loose aggregation of 

 the crystalline grains of carbonate of lime, or of the compound of car- 

 bonates of lime and magnesia, sand, etc.) without intervening cement, 

 or from want of the pressure necessary to consolidate the mass. We 

 have examples of this in the friable marbles and some sandstones. In 

 some cases this condition occurs where the rocks have been much dis- 

 turbed since their deposition and partial or entire consolidation. But 

 this condition as frequently occurs in rocks which, so far as we know, 

 have not been subjected to change, and lie in their original horizontal 

 position. One of the most remarkable examples occurs in the western 

 extension of the Potsdam sandstone, much of wliich, in some parts 

 of Wisconsin and Minnesota, may be easily quarried with pick and 

 shovel, and readily crumbles into an incoherent sand. Above the Pots- 

 dam the St. Peter's sandstone has still less coherence, and is shovelled 

 out in the same manner as the ordinary sand of the drift of the sea 

 beach. From this incoherent condition of the mass, we have all grada- 

 tions to the most strongly coherent rock. This condition of the par- 

 ticles, be it in greater or less degree, affects the strength and durabil- 

 ity of the stone. 



Blocks of stone, wanting proper cohesion, may crack or be partially 

 crushed by superincumbent weight ; but ordinary judgment will guard 

 against using such improper material. The cohesion of the particles 

 or grains composing a stone does not depend upon their hardness or 

 density ; for the grains or crystals composing a mass of marble, and 

 having half the density of grains of sand, often produce a stronger 

 stone than one made up of tiie better material. 



3. Porosity. The porosity of a stone is, in most instances, directly 

 dependent on the degree of cohesion among the particles. Crystalline 

 masses are usually less porous than mechanical aggregations ; and 

 where the interstices between the crystals are filled with a finer 

 material, it has been shown that the latter is porous and absorbent, 



