GEOLOGV 363 



spinel, sillimanite, cordierite, and anorthite are the most 

 frequently occurring minerals. They are due to the permeation 

 of an aluminous sediment by magmatic matter, especially by 

 lime, ferrous iron, and magnesia. 



The alteration is held to be of a deep-seated character, and 

 is believed to have been produced by the reaction of tholeitic 

 magma upon the lining of its basin. The mutual relations of 

 the xenolithic minerals are closely dealt with in the light of 

 recent work on fused mixtures of silicates and oxides. The 

 temperatures concerned in the metamorphism of the xenoliths 

 are shown to have been between 1400° C. and 1250° C. 



Following hard upon Dr. Thomas's investigation of the 

 actual behaviour of certain kinds of xenolith in basic magmas 

 comes Dr. N. L. Bowen's theoretical work on the same subject, 

 based upon the reaction principle, and the observed phenomena 

 in melts of definite composition (" The Behaviour of Inclusions 

 in Igneous Magmas," Journ. GeoL, 30, 1922, pp. 513-70). The 

 two researches are mutually supporting. Bowen shows that 

 in the great majority of cases little or no superheat can be 

 developed in magmas. The exceptions are in basic magmas 

 and at long-lived volcanic centres where exothermic gas 

 reactions may provide some superheat. Hence the phenomena 

 that take place when inclusions are immersed in liquid magma 

 are not solutions in the narrow sense, but chemical reactions 

 between the liquid and solid that mostly follow and do not 

 disturb the normal course of crystallisation. Only the relative 

 proportions of the resulting products are altered. 



An important point which is demonstrated is that a liquid 

 saturated with one member of a reaction series is effectively 

 supersaturated with all preceding members of that series. It 

 cannot dissolve such members if they are added anew to the 

 liquid, but can only react with them to convert them into the 

 members with which it is saturated. Granite magma cannot 

 dissolve olivine, for instance ; but it will convert the olivine 

 into biotite and hornblende, members of the reaction series 

 with which it is saturated. 



The behaviour of igneous and sedimentary inclusions of 

 various kinds in basaltic and granitic magmas is discussed at 

 length. It is concluded that magmas may incorporate con- 

 siderable quantities of foreign matter both by reactive solution 

 and reactive precipitation. It is thought doubtful, however, 

 whether the presence of foreign matter is ever essential to the 

 production of any kind of differentiate. The inclusions may 

 accentuate the normal differentiation tendencies which are 

 possible in a given magma, and which would proceed in any 

 case without the intervention of foreign matter. 



Prof. S. J. Shand's presidential address to the Geological 



