Ixviii PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OI,. 



linite be heated to redness, a sudden and remarkable change takes 

 place. It glows brightly for a few moments, and after cooling is 

 found to have become denser and strongly birefringent. Thus 

 gadolinite occurs in two phases : a lighter isotropic phase, and 

 a denser birefringent phase. The change from the former to the 

 latter takes place at a red heat, and the reaction is accompanied 

 by a considerable loss of energy, but little or no loss of material. 

 The occurrence of this mineral in some granites proves, therefore,, 

 according to Scheerer, that they must have consolidated below a 

 red heat. 



These three lines of argument, based on the presence of quartz^ 

 on the mutual relations of the constituents, and on the presence in 

 some granites of pyrognomic minerals, concur, he considers, in dis- 

 proving the theory of pure igneous fusion. 



Scheerer then propounds his own theory of aqueo-igneous fusion,, 

 basing it on the fact that some of the granitic minerals, such as 

 mica, contain water. The presence of even small quantities of 

 water would, he maintains, lower the consolidation-point consider- 

 ably, and during consolidation the water would concentrate in the^ 

 mother-liquor, and ultimately in the silica. Final consolidation 

 would take place on the escape of the water. Thus the paradoxical 

 order of crystallization would be explained, and the granite might 

 consolidate at a temperature which would admit of the formation 

 of the pyrognomic minerals. 



The theory of Scheerer was opposed by Durocher, and the con- 

 troversy between these two distinguished men extended over a 

 period of three or four years. Durocher considered that a close 

 examination of the structure of granite does not bear out the view 

 that there is a well-defined order of consolidation. The minerals 

 mutually interfere one with the other, sometimes one, and sometimes 

 another, having the advantage. The magma appears to have cooled 

 down to a comparatively low temperature, and then to have sepa- 

 rated into definite compounds which did not solidify instantaneously^ 

 The relative perfection of form would, on this view, be largely 

 determined by the relative power of crystallization of the different 

 constituents. In this respect quartz is at a disadvantage. It 

 possesses, moreover, as shown by M. Gaudin, a great range of 

 viscosity, and when fused can be drawn into threads like glass and 

 sealing-wax. He agrees with Scheerer in rejecting Pournet's theory 

 of surfusion, and considers that the paradoxical order of consolida- 

 tion can be explained by taking into consideration the wide range 



