188 THE CANADIAN NATURALIST. [May 



imprinted themselves upon the lower surface of the solidified flux, 

 a very plain proof being thus furnished that at a temperature of 

 200° R., the flux was still soft enough to receive the impression of 

 the solidifying metal. If we further observe the various fused 

 slags which flow from different furnaces, we shall obtain some idea 

 of the manner in which the rocks above described may have be- 

 haved during their solidification. The scoriae of iron furnaces are 

 usually very acid, containing as much as 60 per cent, of silica. 

 They generally fuse at a temperature of 1450° C. As they flow 

 out of the breast of the furnace, they may be observed to do so 

 very leisurely, to be sluggish and viscid, but nevertheless to con- 

 tinue fluid a long time, and even in some cases to flow out of the 

 building in which they have been produced, before solidifying. 

 On the other hand, slags from certain copper furnaces, or from 

 those used for puddling iron, are more or less basic, containing 

 from 30 to 45 per cent, silica. As they flow out they are seen to 

 be very fluid, and to run quickly, but they solidify much more 

 rapidly than iron slags. Yet these basic slags fuse at about 1300° 

 C, or about 150° less than the more acid slags. Those who have 

 been accustomed to observe metallurgical processes will not find it 

 difficult to conceive how a very siliceous slag might continue fluid 

 at a temperature at which a more basic one might become solid. 

 We conceive, however, that the rocks which we have described 

 must heave solidified under circumstances altogether different from 

 those under which furnace slags cool. We suppose that these 

 rocks must have solidified at temperatures not very far below their 

 fusing points ; that the temperature of the atmosphere, and of the 

 fluid mass itself, had sunk somewhat beneath the fusing point of 

 the more basic rocks before solidification began, and that at this 

 point it was possible for the basic rocks to crystallize, while a more 

 siliceous magma still remained plastic. This latter supposition 

 does not appear unreasonable when the experiment above referred 

 to, and the behavior of furnace slags above described, is taken into 

 consideration. 



It becomes a question of much interest as to whether these rocks 

 are to be regarded as constituting one and the same, or several and 

 distinct, geological formations. There cannot be a doubt as to the 

 fact that some of them are of more recent origin than others ; but, 

 on the other hand, many of the veins above described do not pre- 

 sent such distinct joints as are visible where trap or basalt dykes 

 traverse sedimentary strata. Although the cementing material 



