Ixxvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9OI, 



In the case of the alloys the spherulitic structures are charac- 

 teristic of rapid cooling, and the micropegmatitic structures of 

 slow cooling. The mode of occurrence of the same structures 

 in rocks is strictly in accordance with this view. A comparison 

 of the structures in the two cases makes it almost impossible to 

 believe that the resemblances are merely accidental, and if not, 

 they point to the conclusion that micropegmatite is an eutectic 

 compound. 



From this point of view it becomes of interest to determine 

 the melting-point of fused micropegmatite. This was kindly done 

 for me by Prof. Joly, by observations with the meldometer. He 

 found that fused micropegmatite melted somewhat more readily 

 than orthoclase, but less readily than fused orthoclase. These 

 observations do not support the eutectic hypothesis, but they can 

 scarcely be said to negative it, as the conditions of the experiment 

 are certainly very different from those under which the rocks are 

 produced. 



Quartz and orthoclase have not as yet been formed by pure 

 igneous fusion. The melting-point, at atmospheric pressure, of a 

 mixture of quartz and orthoclase is above that of basalt, and yet we 

 know from the occurrence of angular fragments of basalt in grano- 

 phyre, that the consolidating-point of the mixture under certain 

 conditions of pressure is below that of the fusing-point of basalt 

 under the same conditions. If, as Prof. Loewinson-Lessing's 

 calculations suggest, the formation of felspathic minerals is accom- 

 panied by an increase in volume, and the formation of ferro- 

 magnesiac minerals by a decrease in volume, pressure will lower 

 the fusing-point of the former and raise that of the latter, so that, 

 under plutonic conditions, the relative order of consolidation of acid 

 and basic magmas may be the reverse of that nnder volcanic 

 conditions. Magmas usually contain water, and sometimes other 

 volatile constituents (such as chlorine, boron, fluorine, etc.), whose 

 importance in determining the fluidity and the molecular grouping 

 of the constituents has been generally recognized since the publica- 

 tion of the classic paper ' Sur les Emanations Yolcaniques & Metal- 

 liferes ' by Elie de Beaumont. When separated from the magma, 

 these constituents exercise most important metamorphosing and 

 mineralizing effects, as is well seen in the phenomena accompanying 

 the formation of tinstone and apatite-veins, in the development of 

 zeolites, and in the production of large masses of kaolin. But, so 



