ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 159 



ing vapours which remained after the crystaUisation of the major 

 part of the non-metalhc constituents of the magma with which 

 this paper deals. 



The precipitation of the metalhc contents from different 

 centres will therefore depend almost wholly upon the physico- 

 chemical conditions obtaining at different points along the paths 

 of emergence of the expelled vapours^. 



If we follow for the moment the emerging vapours from one 

 segregation centre along a fissure-system extending into higher 

 portions of the crust, the precipitation of the metalhc ores will 

 depend upon : — 



(1) The temperature of the rocks traversed. The further 



the metalliferous solutions travel from their source and 

 from the mantle of hot igneous rock enveloping the 

 source the lower their temperature becomes, by loss of 

 heat to the wall rocks. 



(2) The pressure at points along their course. The pressure 



to which the solutions are subject decreases continually 

 with their ascent. 



(3) The rate of migration from the hearth. The ease or 



difficulty with which the expelled materials are able to 

 ascend will materially influence precipitation. Where 

 the upward path is free and open the migration will be 

 rapid, and the solutions will carry tlieir materials 

 further before the conditions of precipitation are 

 reached. Where the path is restricted and complex, 

 precipitation will be brought about more quickly and 

 at a less distance from the source, mainly on accovmt of 

 the inevitable loss of temperature which the solutions 

 will suffer under such conditions. 



(4) The thermal conductivity of the several rocks traversed. 



This will play some part, perhaps unimportant, in 

 inducing precipitation. 



(5) The physical characters of the rocks, and consequently 



the stmctural features of the fissures traversing them. 



Of these conditions which control precipitation it appears to 

 the author that the most important by far is the first. The 

 temperature acquired by the escaping materials is the main deciding 

 cause for the deposition of the mineral load ; and the other condi- 

 tions, save that regarding pressure, are of importance, in that they 

 decide the particular points at which the temperature necessary 

 for precipitation is attained. Thus, if the temperature has almost 

 reached the point at which the metalliferous solutions become 

 supersaturated, and if a retardation in the rate of migration takes 

 place, the temperature will fall sufficiently to enable precipitation 

 to begin at that part of the fissure-system. 



Thus far we have considered the simple case of the disposition 

 of mineral material of one kind in one fissure system which is con- 

 nected with a single segregation centre. When several metals are 



1 The author desires to acknowledge his indebtedness to the writings of W. H. Weed and others 

 for many of the conceptions involved in this discussion. 



