424 Rejjorts and Proceedings — British Association. 



An admirable summary of the evolution of petrographical ideas 

 was given in the Presidential Address to the Geological Society of 

 London in 1901 by Dr. Teall, who dealt both with the consolidation 

 of rocks from molten magmas and their differentiation into species. 

 It is not, therefore, necessary for me to consider anything but recent 

 work which has been done during the last four years, and the earlier 

 controversies may be left out of account. 



Among the many problems relating to the mineral and chemical 

 constitution of rocks which have yet to be solved, two, and those 

 perhaps the most important, should lend themselves most readily to 

 experimental treatment. The first is the problem of rock differentia- 

 tion : why does a magma, even one which has presumably crystallised 

 in situ, separate itself into zones, or layers, or streaks of different 

 constitution ? And the second is the problem of mineral differentia- 

 tion : why does a granite magma, for example, crystallise as a mixture 

 of the particular minerals mica, felspar, and quartz, and why is the 

 least fusible mineral the last to crystallise ? 



It will scarcely be possible for me to deal in this Address with 

 more than the second of the two problems, but it will be apparent 

 from the somewhat parallel case of the salt deposits that the mere 

 order and manner of crystallisation of a mass of molten silicates 

 must be a sufficiently complex problem to exhaust our attention for 

 the present. 



Magmattc Differentiation. 



If we are to consider only recent experiments which have a bearing 

 upon the problems of rock-magmas, it is not necessary to say much 

 about the first great petrographical problem, that of the differentiation 

 of magmas into various rock types ; for in this connection very few 

 experiments have been made, and practically none of recent date. 

 Observations of the facts as they present themselves in the field 

 accumulate every day ; almost every important petrographical region 

 is being studied with the particular object of determining the mutual 

 relations of its rock-masses and the factors which have contributed 

 to their differentiation. They have been ably discussed by Becke, 

 Brogger, Becker, Cole, Harker, Iddings, Judd, Lacroix, Levy, Pirsson, 

 Kosenbusch, Teall, Washington, Zirkel, and many others ; appeal 

 has been made to the action of gravity, of temperature differences, 

 of diffusion, of electric currents, of fractional crystallisation, of 

 refusion, of chemically combined water, of absorption of the country 

 rock ; but with the exception of a single case, observed in the 

 glassworks of Targowek, in which the top of a molten glass was 

 found to contain less lime and more silica than the bottom, and some 

 observations by Doelter upon boron-glass, there is scarcely a single 

 experiment upon silicates which really bears directly on the question. 

 That artificial glasses are far from homogeneous is known to glass- 

 workers and to makers of lenses, but there is nothing comparable 

 with the splitting of a magma into two or three distinct liquids which 

 solidify as different rocks. 



It is in the case of laccolites that the problem ought to present 

 itself in the simplest form, for we may regard them as basins of 



