42 



NEW YORK STATE MUSEUM 



Absorption. The absorption of a rock is the ratio between the 

 weight of the absorbed water and the dry weight of the sample. It 

 is determinable, therefore, from the same measurements that are 

 used in finding the porosity. The weight of the absorbed water is 

 divided by the weight of the dry stone; the result multiplied by lOO 

 gives absorption as a percentage of the mass. The relation between 

 porosity and absorption varies with the specific gravity of the stone, 

 but the latter commonly amounts to about one-half of the former. 



The ratio of absorption, any more than the porosity, does not 

 afiford an absolute index of the permeability of stone to water. 

 Parks ^ has conducted an interesting experiment to test the perme- 

 ability in samples having different porosities. Samples of rock 3 mm 

 thick were cut at right angles to the bedding planes. Through these 

 pieces water was forced under pressure of 15 pounds to the square 

 inch and the amount of flow in one hour recorded. It was found 

 that stones having less than i per cent of pore space were prac- 

 tically impermeable to water under that pressure. The results on 

 some sedimentary rocks are as follows : 



POROSITY 

 PER CENT 



permeability: 



CU. CM OF 

 WATER AN HOUR 



Guelph limestone 



Guelph limestone 



Chazy lime'stone 



Medina sandstone 



Niagara limestone 



Beekmantcwn limestone 

 Potsdam sandstone 



IS. 883 

 14.62 

 17.517 

 10.44 

 10.443 

 1.313 

 4-947 



90. S 

 iSS.i 



Hardness and toughness. Hardness is a property of homo- 

 geneous materials like minerals by which they resist penetration. It 

 lacks the same degree of definiteness when applied to rocks which 

 are composed of various minerals and perhaps held together by 

 some cementing substance of still different nature. In such condi- 

 tions, it may be regarded as the resultant of the hardness of the 

 various ingredients plus the bond betv/een them. 



There is no uniformity in the practice of determining hardness, 

 which is an important feature of materials to be used in paving and 

 street work generally. One method follows that in use for compar- 

 ing the hardness of minerals and is based on the rate of penetration 



1 " Building and Ornamental Stones of Canada, Ottawa, 1912," v. i, p. 61-62.. 



