402 GEOLOGY. 



of snow, sugar, and salt be mixed at a low temperature and compacted, 

 the mass may be regarded as an artificial rock. On raising the tem- 

 perature, all will pass into solution while the temperature is still some- 

 what below the melting-point of the snow, the most fusible, and while 

 it is much below that of either the sugar or the salt. This particular 

 case is instructive because the ice is not simply fused by tempera- 

 ture; the affinity of the salt plays a part. If the temperature were 

 again lowered, the sugar and salt would not crystallize out at their 

 fusing-points, but would remain in solution down to and even below 

 the normal freezing-point of water; in other words, they would remain 

 in solution until the water crystallized out and forced them to take 

 the solid state. This holds good when the amounts of the sugar aiid 

 salt are small relative to the water. If, on the contrary, the!r quantity 

 is large relatively, crystallization will take place at h'gher temperatures 

 and before the water crystalUzes to ice. From this it appears that 

 the salt and sugar might crystallize either before the water or after it, 

 according to the degree of concentration. The behavior of mixtures of 

 minerals in passing into and out of the molten condition appears to be 

 quite analogous to this, and hence a great variety of results attend the 

 process, dependent upon the number, the nature, and the relative 

 quantities of the ingredients. The approved conception of the genesis 

 of a rock from a molten magma (when ample time is given) is that one 

 compound after another crystaUizes out as the temperature falls and 

 its point of saturation for each is reached, until the whole has been 

 solidified. The modes of combination of the elements in the molten 

 magma are not necessarily the same as those in the derivative crystals; 

 indeed, the combinations doubtless change as the process proceeds; 

 certain constituents being taken out, the remaining ones probably 

 rearrange themselves. 



Time required in crystallization. — The liquid magma of igneous rocks 

 is essentially a fluid glass or slag. It is analogous to common glass, 

 which is a sihcate of potash, soda, or other base, except that usually 

 common glass is relatively free from iron and other coloring substances, 

 while these abound in the natural magmas and render them dark and 

 more or less opaque; but the fundamental nature is the same, except 

 that the natural lavas are usually mixtures of several siHcates, while 

 the artificial glasses consist of only one, or at most a few. Furnace 

 slag is essentially an artificial lava. 



