164 POROSITY OF ROCKS. [IX. 



APPENDIX. 



ON THE POROSITY OF ROCKS. 

 [From the Report of the Geological Survey of Canada for 1863-66, pages 281-283.] 



ALL rocks are more or less porous, and most uncrystalline sedi- 

 mentary ones possess this character to a very considerable degree. 

 Such rocks when taken from the quarries are more or less com- 

 pletely saturated with water, from which, indeed, they have never 

 been free since the time of their formation. This water they gradu- 

 ally lose when exposed to the air, and, as is well known in the case 

 of many building-stones, become much harder than before. The 

 porosity of rocks is of considerable importance in relation to their 

 value as building materials. The open spaces between the particles 

 diminish the cohesion of the mass, and, in addition to this, the water 

 held in the pores of a rock, when exposed to cold, tends, by its ex- 

 pansion in freezing, to disintegrate the mass, and cause it to crumble, 

 a consideration of much importance in a cold climate. Other things 

 being equal, it may probably be said that the value of a stone for 

 building purposes is inversely as its porosity or absorbing power. 



The study of the porosity of rocks is, moreover, of much interest 

 from a geological point of view. As I have elsewhere endeavored 

 to show (ante, pages 103 and 163), the origin of most of the muriated 

 saline springs is to be sought in old sea- waters and bitterns impris- 

 oned in ancient sedimentary strata, which must now hold in their 

 pores an amount of water bearing a considerable proportion to 

 the entire volume of the present ocean. The observations here 

 given were made in 1864, with reference to both of the above 

 considerations. 



The method of investigation was as follows : Small broken frag- 

 ments of the rocks generally from twenty to forty grammes in 

 weight were selected, and freed from scales or loose grains, which 

 might, by falling off during the experiment, vitiate the results. 

 These specimens were carefully dried at about 200 F., till they 

 ceased to lose weight ; most of them had, however, been long pre- 

 served in a dry room, and were found to be. nearly free from moist- 

 ure. The weight of these having been determined, they were placed 

 with their lower portions in water, and allowed to remain for some 

 hours, after which they were covered with water and placed under 



