IX.] POROSITY OF ROCKS. 167 



The rocks in the preceding table, with, the exception of six, are 

 from the palaeozoic formations of Canada, including, as will be seen, 

 pure limestones of the Trenton formation, dolomites of the Calcifer- 

 ous sand-rock, the Chazy, the Onondaga (or Salina), the Niagara, and 

 the Guelph, a local formation resting upon the Niagara. The sand- 

 stones are from the Potsdam, the Medina, and the Sillery, a mem- 

 ber of the Quebec group, which is associated with the argillaceous 

 shale No. 15, with which are compared the argillaceous shale of the 

 Hudson River group and the compact pyroschists of the Utica 

 formation. I have given in Nos. 12, 13, and 14 determinations with 

 three specimens of a fine gray and very porous sandstone from 

 Ohio, of Devonian or Lower Carboniferous age, and much used for 

 building. Nos. 37, 38, and 39 are three specimens of the well-known 

 soft limestone of Caen, in France, so much employed in that country 

 for architectural purposes. 



10.9 ; for four nearly pure limestones from the oolite, 18.0, 20.6, 24.4, 31.0; 

 for four magnesian limestones, 18.2, 23.9, 24.9, 26.7; and for six sand- 

 stones, 10.7, 11.2, 14.3, 15.6, 17.4, and 22.1. These numbers represent 

 the absorption obtained by the aid of the air-pump, without which it is 

 impossible to remove all the air from the pores of the previously dried 

 rock. Thus a cube of two inches of a sandstone which takes up in this 

 way 14.3 of water only absorbed 8.0 by prolonged immersion in water; an 

 oolitic limestone, capable of holding 20.6, in like manner absorbed only 

 13.5 ; and a magnesian limestone only 9.1, instead of 24.4. (See, also, On the 

 Porosity of Rocks, Delesse, Bull. Soc. Geol. de France (2), XIX. 64.) 



