Fluid Cavities in Qiiartz. 53 



an amorphous form. Therefore it is said that it was 

 probably held in partial solution in hot water, even after 

 crystallisation by segregation of the other minerals had 

 begun. This theory, now held by several distinguished 

 physical and chemical geologists, seems to me to be 

 sound, especially as it agrees exceedingly well with the 

 metamorphic theory as applied to gneiss granite being 

 sometimes simply the result of the extreme of meta- 

 morphism. In other words, when the metamorphism 

 has been so great that all traces of the semi-crystalline 

 laminated structure has disappeared, a more perfect 

 crystallisation has taken place, and the result is a 

 granitic mass without any minor lamination in it. Even 

 then, however, certain planes often remain, strongly 

 suggestive of original stratification, and even of planes 

 of oblique stratification or false-bedding. 



These general results are not founded on mere conjec- 

 tures. In a memoir by Mr. H. C. Sorby, 'On the Micro- 

 scopical Structure of Crystals, indicating the Origin of 

 Minerals and Rocks,' among other important points, he 

 describes numerous small cavities in the quartz of granites, 

 which are partly filled with water ' holding in solution 

 the chlorides of potassium and sodium, the sulphates 

 of potash, soda, and lime, sometimes one and sometimes 

 the other salt predominating.' These ' fluid cavities ' 

 sometimes make up about five per cent, of the volume 

 of the quartz, and he concludes that 'the fluid was not 

 an accidental ingredient due to the percolation of water 

 to a fused mass naturally containing none, but & genuine 

 constituent of the rock when melted.' Reasoning on the 

 underground temperatures necessary to expand the 

 liquid so as to fill the cavities, by an elaborate process 

 of argument he arrives at the approximate result, that 

 ' the pressures under which granites were most probably 



