1867.] MACPARLANB — GEOLOGY OF LAKE SUPERIOR. 187 



been produced. If, on the other hand, the solidification took place 

 while the fluid mass was in motion, the hornblendic and micaceous 

 schists and gneisses were most probably the results of this process, 

 and the strike of these would indicate the direction of the current 

 at the time of their formation. The rarity or indistinctness of 

 parallelism in the Laurentian rocks of Lake Superior shews, how- 

 ever, that no very constant and persistent motion in one direction 

 took place in the fluid mass which produced them. This first 

 solidification of part of the fluid magma most likely continued for 

 a long period, and spread over a large surface ; but there seems 

 at last to have arrived a time when, from some cause or other, 

 these first rocks became rent or broken up, and the crevices or 

 interstices became filled with the still fluid and more siliceous 

 material which existed beneath them. Gradually, this material 

 solidified in the cracks, or in the spaces surrounding the fragments, 

 and the whole became again a consolidated crust above a fluid mass 

 of still more siliceous material. Further solidification of this 

 latter material doubtless then took place, and continued until a 

 second general movement of the solidified crust opened other and 

 newer crevices, which became filled with the most siliceous ma- 

 terial which we see constituting the newer veins among the rocks 

 above described. 



Although the theory here given as to the origin of these rock 

 aggregates is in thorough harmony with the facts related concern- 

 ing them, it is doubtless possible to urge objections against it 

 founded upon the relative fusibility of their constituent rocks. 

 There is no doubt that the point of temperature at which these 

 various rocks become fluid under the influence of heat is higher 

 with the newer than with the older rocks, but it does not follow 

 that in cooling they solidify, that is, become quite hard and solid 

 at the same point of temperature at which they fuse. Bischof 

 describes an experiment which proves that the temperature at 

 which certain substances solidify does not at all correspond with 

 their fusing point. He prepared a flux, consisting of common 

 glass and carbonate of potash, which fused at a temperature of 

 800° E,., and melted it along with some metallic bismuth in a 

 crucible. This metal fuses at 200°, and solidifies with a very 

 uneven surface, on account of its tendency to crystallize. Although 

 the difference between the fusing point of the bismuth and of the 

 flux amounted to 600°, nevertheless, when the crucible cooled, all 

 the irregularities of the surface of the metal were found to have 



