362 SCIENCE PROGRESS 



In a paper on the development of pressure in magmas as 

 a result of crystallisation, G. W. Morey {Journ. Wash. Acad. 

 Sci., 12, 1922, pp. 219-30) shows that in a system such as KNO3 

 and water, consisting of volatile and non-volatile constituents, 

 crystallisation takes place at a temperature lower than the 

 f.p. of the pure non-volatile salt by an amount corresponding 

 to the quantity of volatile substance present, and that the 

 three-phase pressure increases rapidly as the temperature is 

 lowered. Hence, in magmas consisting of silicates and water, 

 in which considerable pressure is required to retain the water 

 in solution, a small amount of crystallisation will result in a 

 large increase of pressure. Under suitable conditions a volcanic 

 outburst might take place as a result of this development of 

 pressure. The production of a " bursting pressure " through 

 crystalhsation in magmas was premised in W. H. Goodchild's 

 memoir entitled " The Evolution of Ore Deposits from Igneous 

 Magmas," pubhshed in the Alining Magazine (London) for 191 8, 

 a paper of outstanding importance outlining bold petrogenic 

 theories, which have, however, attracted little attention in 

 this country, and apparently none at all in America (see Science 

 Progress, January 1919, p. 375)- 



Pure artificial orthoclase has been prepared by G. W. 

 Morey and N. L. Bowen {Amer. Journ. Sci., (5), 4, 1922, pp. 1-2 1 ). 

 With the use of this material orthoclase has been shown to be 

 an example of incongruent melting, breaking up at ii70°C. 

 into leucite and a hquid. As the temperature is raised leucite 

 finally disappears at iS30°C. Thus the interval of incon- 

 gruent melting is very large, viz. 360° C. The establishment 

 of this fact is of great petrogenetic importance, since it shows 

 that under suitable conditions leucite can separate from an 

 orthoclase melt, which may later on deposit free silica. If 

 the early leucite crystals are removed from immediate contact 

 with the liquid by any means, it will become possible for 

 phenocrystic leucite to exist in a ground mass containing quartz, 

 as in the leucite-granite-porphyry of Brazil. Furthermore, 

 this phenomenon explains the clear gravitational association 

 of leucite rocks with quartz-syenite, etc., in such masses as 

 that of L. Borolan, Sutherlandshire. 



Dr. H. H. Thomas has fully described the xenolithic material 

 that occurs in some tholeitic minor intrusions of Tertiary age 

 in Mull {Quart. Journ. Geol. Soc, 78, pt. 3, 1922, pp. 229-60). 

 Both cognate and accidental inclusions are present ; the former 

 consist of bytownite and pyroxene aggregates ; the latter are 

 of sihceous and aluminous composition. The sihceous xenoliths 

 are characterised by the development of tridymite ; but the 

 most complex and interesting mineralogical developments are 

 found in the aluminous xenoliths. Corundum (sapphire), 



