158 PROCEEDINGS OF SECTION C. 



the district. The differences of this kind in the distribution of the 

 segregated metals will, in general, be more marked in the case of 

 large areas than in the case of small ones. 



Different fissm"es, being connected in depth with different 

 centres of segregation, may therefore be filled with different metals 

 or with the same metals in different proportions. 



The fissures which have connection with the centres of segre- 

 gation offer the means whereby the imprisoned vapours are enabled 

 to ascend into the upper portions of the crust. During this upward 

 passage precipitation takes place, and the minerals of the primary 

 ore-bodies are formed. It is probable that the metallic compounds 

 at the moment of leaving the magmatic hearth existed in the 

 gaseous form, but with the fall of temperature and pressure which 

 they experience during their upward migration, they attain the 

 solid form, or pass into solution in the complex solutions formed 

 by the condensation of the several vapours when the temperature 

 and pressure become low enough to admit of the existence of these 

 in liquid form. 



From these solutions the dissolved metallic compounds are 

 precipitated, according to definite physico-chemical laws. The 

 most important immediate causes of precipitation are the condi- 

 tions of temperature and pressure, while these are modified by the 

 chemical composition of the solutions {i.e., by their dissolving 

 power). 



It appears to the author most probable that the emerging 

 vapours, issuing from the several centres of segregation within a 

 cooling magma such as that which we have been considering, will 

 at the moment of their expulsion have a temperature which is 

 sensibly constant. For, if the hypothesis outlined above with 

 regard to their genesis is correct, they represent the residues left 

 after the solidification of certain definite rock types. In the 

 consideration of this point, it is essential to remember that the 

 temperature referred to is not that of the fusion of the dry rock. 

 From the foregoing discussion it is plain that the solidification 

 of the aplites and pegmatites takes place in the presence of 

 abundant vapours, which have the effect of very materially lowering 

 the freezing point of such rocks. Working on the lines suggested 

 by Messrs. F. E. Wright and E. S. Larsen^, it has been found by 

 Mr. E. S. Bastin that the temperature of solidification of the 

 pegmatites of Maine, U.S.A., lies between 560°C. and 580°C.2 

 How far varying proportions of different mineralizing agents are 

 capable of altering the temperature of solidification of pegmatites 

 is not yet known. By the comparison of Mr. Bastin's results 

 with those obtainable from the pegmatites of different regions, 

 much valuable information is to be obtained. If the conclusions 

 arrived at by Mr. Bastin with regard to the Maine pegmatites 

 prove upon investigation to be of universal application, we are in 

 a position to ascertain the maximum temperature of the mineraliz- 



1 "Quartz as a Geologic Thermometer." American Journal Science, June, 1909, pp. 423-447* 



2 " Origin of the Pegmatites of Maine." Journ. Gcol., May-June, 1910, pp. 297-320. 



