CRYSTALLIZATION OF MAGMA 1 93 



will dissolve the copper at a temperature far below that at which 

 the copper would melt alone. 



In a rock magma the crystallization of the more and more 

 soluble minerals will proceed regularly, provided the pressure and 

 rate of cooling continue constant. As these conditions are, how- 

 ever, subject to variation, it frequently happens that the more 

 soluble minerals begin to crystallize before the less soluble have 

 all been formed, and thus the periods of formation of two or more 

 kinds of minerals partly overlap. 



Usually, the mode of formation of the different kinds of min- 

 erals in a solidifying mass is as follows. First to form are apatite, 

 the metallic oxides (magnetite, ilmenite) and sulphides (pyrites), 

 zircon, and titanite. " Next come the ferro-magnesian silicates, 

 olivine, biotite, the pyroxenes, and hornblende. Next follow the 

 felspars and felspathoids, nepheline and leucite, but their period 

 often laps well back into that of the ferro-magnesian group. Last 

 of all, if excess of silica remains, it yields quartz. In the variations 

 of pressure and temperature, it may and often does happen that 

 crystals are again redissolved, or resorbed, as it is called, and it 

 may also happen that after one series of minerals, usually of 

 large size and intratelluric origin, have formed, the series is again 

 repeated on a small scale, as far back as the ferro-magnesian 

 silicates. Minerals of a so-called second generation thus result, 

 but they are always much smaller than the phenocrysts and are 

 characteristic of the ground mass. 



" It results from what has been said that the residual magma is 

 increasingly siliceous up to the final consolidation, for the earliest 

 crystallizations are largely pure oxides. It is also a striking fact 

 that the least fusible minerals, the felspars and quartz, are the 

 last to crystallize." (Kemp.) 



A very considerable number of minerals are found in the 

 igneous rocks, but comparatively few in any large quantity. It 

 thus becomes necessary to distinguish between the essential min- 

 erals of a rock and the accessory ones. The essential minerals are 

 those the presence of which is necessary to the formation of a 

 given kind of rock, while the accessory minerals are those which 



