MARYLAND GEOLOGICAL SURVEY 201 



nificant in quantity and so stable under atmospheric conditions that 

 they do not influence appreciably the physical or chemical stability of 

 the rock. 



In any discussion or consideration of building stones, in order to 

 appreciate the practicability of the rocks for large and permanent 

 structures, it is necessary to know something of their physical proper- 

 ties. Among these the most important are specific gravity, the ratio 

 of absorption, the effect of freezing and thawing, and the compression 

 strength. The specific gravity must be known in order to compute 

 the weight to each cubic foot of the rock, which in turn indicates the 

 amount of pressure imposed on the lower courses of the structure. 

 Since almost all building stones are exposed to the atmospheric agents 

 which influence them, it is well to know also what effect the varying 

 conditions of temperature have upon a given stone. For example 

 heating, due to the rays of the sun, causes the minerals to expand. 

 Since the rate of such expansion is different for different minerals and 

 even for different directions in the same mineral, there is unequal en- 

 largement of the grains, and hence a loss in the cohesive strength of 

 the rock. Other things being equal this change is greater in aggre- 

 gates composed of many and vari-colored constituents. Again, if the 

 rock is porous, the expansion of included moisture may rend the rock 

 in freezing weather, thus it becomes necessary to know the amount 

 of moisture absorbed by the rock, and so liable to expansion through 

 frost action. The values obtained by Gillmore x on Port Deposit 

 granite are as follows: 



Weight Eatio 



Strength Strength ofl of 



of per cubic absorp- 



Position. Cracked, spec. sq. in. Sp. gr. ft. tion. Remarks. 



On bed 79,000 19,750 2.720 170 Coarse, strongly dashed with black. 



On edge, 33,000 52,400 13,100 2.720 170 do 



On bed 66,000 16,500 2.720 170 do 



« « 60,000 15,000 2.720 170 Burst suddenly. 



In the tests made during the search for a stone suitable to be used 

 in the building of the Smithsonian Institution at Washington several 



iGillmore, Reports on the Compressive Strength, Specific Gravity and Ratio of 

 Absorption of the Building Stones in the United States. Rept. of the Chief of 

 Engineers for 1875, Appendix II, p. 847. Also Republished 8vo. 37 pp. Van Nost- 

 rand, New York, 1876. 



