160 THE CANADIAN NATURALIST. [June 



they present merely the appearance of natural glass. Others, 

 have had time to lay aside the vitreous character and assume a, 

 stony appearance, but they appear so homogeneous and fine- 

 grained that their compound nature would scarcely be suspected. 

 This is, for instance, the case with basalt, which, on this account, 

 was at one time regarded as a simple mineral. On grinding it to 

 powder and washing it, however, Cordier found it to consist of 

 several minerals with distinct physical characters. A good many 

 other rocks are seen, on examination, to be distinctly compound, 

 but their constituent minerals are developed in such minute 

 grains that their determination becomes a matter of very great 

 difficulty. It is only in the coarser and large grained rocks that 

 the constituent minerals can be readily recognized by the student, 

 and their physical and chemical properties easily tested. 



These variations in the size of the constituent minerals aro 

 accompanied by differences in their form and position, and, both 

 tof^ether, give rise to what is called the texture of crystalline 

 rocks, — difference in which may easily and at once be detected 

 by the student. Coarse and fine grained, schistose and slaty, 

 vitreous, porous, and other such names, are used for characteriz- 

 ing peculiarities of texture, which are not at all to be regarded 

 as merely trifling accidents in the history of rocks, but which 

 really possess a deeper meaning than we are inclined at first to 

 imagine. Although neither the furnace nor the volcano can give 

 us any conception of the magnitude of the scale upon which the 

 earlier original, or, as they have been named, the plutonic rocks, 

 were erupted, still, they furnish us with hints which we cannot 

 afford to neglect. To the metallurgist, it is an every day occur- 

 rence to observe that the same scorioD yields either a vitreous slag 

 or a stony mass, accordingly as it has been quickly or slowly 

 cooled. Slag cakes, a few inches in diameter, are found to be 

 impalpable or glassy on the outside, while on breaking them, the 

 interior is found to be porcelain-like or crystalline. Bischof made 

 some interesting experiments on this matter at the iron-works of 

 Magdesprung in the Hartz. He allowed common iron furnace 

 slag to run into cold water, where it disengaged sulphuretted 

 hydrogen, and yielded a white, easily friable pumice stone. He 

 next allowed the slag to solidify upon cold, somewhat moist, sand. 

 This gave a harder pumice, still retaining some of the original 

 color of the slag. In the next experiment the slag was allowed 

 to cool on a completely dry bottom of sand, and the result was a 



