Notices of Memoirs — Prof. J, Joly — Viscous Fusion. 475 



2. Mica-schists, more or less associated with quartz-schists. 



The first group are, he observes, usually attributed to the 

 alteration of basic augitic rock ; that is, they were originally com- 

 posed of anhydrous minerals, pyroxenes, and felspars. He then 

 adduces evidence to show that they now have become transformed 

 into rock, consisting of hornblende, epidote, zoisite, and chlorite; 

 all of which contain water of crystallisation, together with albite, 

 which is charged with actual water. In short, he remarks that the 

 one characteristic mineral which by way of addition distinguishes 

 the green rocks from their assumed parents is water. 



He then deals with the second group, showing a similar result. 

 Thus, he maintains, the hydro-metamorphism of the district seems 

 pretty well established, and he contends that whilst thermo-dynamic 

 metamorphism is admitted in South Devon, the universal presence 

 of water in the newly constituted rocks must compel us to assign 

 a very important position to hydro-metamorphism as an agent of 

 change. It is not so much that water must have been present 

 during the metamorphio process, as that it is in the rocks now, 

 and could not have been introduced since their crystallisation. 



VII.— On the Viscous Fusion of Eook-forming Minerals. By 

 Professor J. Joly, D.Sc, F.E.S., F.G.S.i 



IN a paper read at the Congres Geologique International of 1900, 

 and in a short note communicated to the British Association 

 Meeting at Bradford, 1900, experiments on the viscous fusion of 

 some rock -forming minerals are described by the author. It 

 appeared that under the influence of prolonged exposure to high 

 temperatures, rounding and other signs of fusion (a breakdown of 

 stability as a solid) could be obtained at much lower temperatures 

 than have hitherto been assigned as the melting - points. This 

 lowering of the melting-point under prolonged heating (four hours) 

 is more marked generally in the case of minerals containing a large 

 percentage of silica, and most marked in the case of quartz. On this 

 account the order of the melting-points is in general different under 

 conditions of prolonged heating than under conditions of short 

 exposure. 



The former results, as regards melting under conditions of short 

 exposure (results required for comparison with those obtained under 

 conditions of long exposure), were not quite satisfactory (as at the 

 time the author pointed out) in so far that the observations were 

 not effected in the same manner as those under conditions of pro- 

 longed exposure. This defect in the observation has now been 

 removed. One-minute exposures have been made of all the minerals 

 previously dealt with on exposures of four hours, and the results 

 confirm the former conclusions, but reveal a decreased difference in 

 the two melting-points ; in other words, the short-exposure melting- 

 points being below previous results, the depression of melting-point 



1 Abstract of a paper read before the British Association, Belfast, Sept., 1902, in 

 Section C (Geology). 



