QUARRY MATERIALS OF NEW YORK 20, 



influences. Tests show that a fine-grained granite may be as porous 

 as a coarse-grained one, which is also true of a sandstone. There 

 is usually a difference in the size of the pores, which are larger 

 but less numerous in the coarser stones ; consequently, it may be 

 said that these will usually absorb moisture more readily and on 

 the other hand dry out more quickly than similar rocks composed 

 of particles in a fine state of division. Whether they weather more 

 or less rapidly than their fine-grained equivalents, depends upon 

 other factors such as the state of aggregation and relative spacing of 

 the particles and the character of the climate. 



Experiments with the St Lawrence and Jefferson county granites 

 indicate that the coarser grades, which contain feldspars up to an 

 inch in diameter, are as closely textured as the fine sorts. There 

 is also no appreciable difference in the two kinds with regard to 

 weathering, so far as can be estimated from the condition of the 

 rocks in natural exposures. 



Crystalline rocks which have consolidated at depths show little 

 porosity, and the variations between different examples are often 

 too slight to have significance for practical purposes. xA.ny marked 

 departure from the average is traceable to external influences in 

 the way of chemical or mechanical disintegration and should be an 

 occasion for careful investigation. 



The fragmental rocks like sandstone and grits are apt to have 

 more pore space. But a degree of porosity above the average is 

 indicative of imperfect cementation. It denotes, therefore, pervi- 

 ousness to moisture, as well as inferior strength through lack of 

 bond. Limestones and marbles may be quite as impervious as the 

 igneous rocks. Porosity in their case may arise from solution by 

 the seepage of underground waters, forming cavities which weaken 

 their structure and not infrequently contain secondary deposits 

 of iron sulphides. 



Apart from these considerations, the size of grain seems to bear 

 some relation to the strength of certain rocks. This has been 

 noted by Julien, 1 who instances the minutely crystalline limestones 

 as examples which may show surprising resistance to crushing ; in 

 a limestone from Lake Champlain the ultimate strength reached 

 25,000 pounds to the square inch. The explanation for the superior 

 strength of such rocks, as given by that writer, is that the molecular 

 cohesion between the grains, under equal conditions, is proportion- 



1 Building Stones — Elements of Strength in their Constitution and Struc- 

 ture. Journal of the Franklin Institute, v. 147. April 1899. 



