Centre Valley Tin Ore 



a little water, the relative quantities in each portion 

 depending upon the temperature. 



Both the above cases appear to occur in igneous 

 magmas ; thus, so far as we know, the various silicates 

 when in the molten condition can mix in any proportions, 

 but the same would not appear to be true of molten 

 silicates and molten sulphides, which separate into 

 separate liquids as the temperature of the magma 

 falls. 



There is a somewhat common association of a certain 

 basic igneous rock called Norite, with the mineral Pyrrho 

 tite, a sulphide of iron usually with some nickel. The 

 ore nearly always occurs for the most part at the bottom 

 of the igneous rock, and as it is much heavier than the 

 molten silicates, its concentration as a separate liquid 

 during cooling would account for this. 



If several crystalline substances be dissolved together 

 in hot water compounds which have no chemical action 

 upon each other being chosen and the solution be 

 slowly cooled, it will be found that the bodies will 

 separate in a definite order, which will depend to some 

 extent on the proportions of the various substances 

 present, for it must be remembered that the presence of 

 one substance in solution profoundly alters the degree 

 of solubility of another. It must also be remembered 

 that many substances which are gaseous at ordinary 

 temperatures will dissolve in liquids, and so become 

 themselves liquid. 



It will thus be seen that the cooling of an igneous 

 magma and the consequent formation of an igneous rock 

 is a by no means simple matter, and that a very careful 

 study of the rock mass is necessary before it is possible 



281 



