Reports and Proceedings — British Association. 473 



Did space admit we might pursue tlie description of this interesting 

 ■exhibition of "Old Masters," but anyone for a modest threepence may- 

 acquire a very carefully prepared introduction to a most remarkable 

 series of works by early authors, some of whose writings and accom- 

 panying illustrations will well repay a visit to that most attractive 

 Natural History Museum in Cromwell Eoad, which has now become 

 one of the most valuable teaching-centres in London, and whose library- 

 is also one of the best extant, a result largely due to the energy and 

 ability of its Librarian, Mr. B. B. Woodward, to whom we are also 

 indebted for the present exhibition of old Natural History books and 

 for one of the best Library Catalogues (still in progress) which has 

 ■ever been printed of works bearing on Natural History. 



:r:h3:f'o:rts .A-IsTid les-OGEiiEnDin^rG-s. 



British Association for the Advancement op Science. 

 Cape Town, South Africa, August 16th, 1905. 



Address to the Geological Section (C) by Professor H. A. Miers, M.A., 

 D.Sc, F.E.S., President of the Section. 



{Continued from the September Number, p. 429.) 



jDoelter's Work on Melting -Faints and Solubilities. 



THE labours of Doelter and his pupils have been largely devoted 

 to the melting-points of the rock-forming minerals and their 

 flolubility in silicate magmas. From experiments upon these minerals 

 and their mixtures they have come to the conclusion that in many- 

 cases the melting-point of the mixture is about the mean of the 

 melting-points of the constituents, and that in such cases, therefore, 

 there is no evidence that the freezing-point is lowered, or that an 

 eutectic mixture is formed ; so that it is not safe to apply the theory 

 of cyro-hydrates to fused mixtures of silicates. 



Doelter is therefore led to regard the silicate magmas rather as 

 mixtures of various constituents which may be dissolved in each 

 other, but which are not by any means necessarily identical with the 

 minerals which separate on cooling. The whole process seems to 

 him to be far too complicated to be explained by any such simple 

 principle as the mere relative proportions of the various constituents 

 to each other and to their eutectic mixture ; the order of crystallisa- 

 tion must be determined by a number of factors, such as temperature, 

 velocity of crystallisation, the interval between the softening and 

 fusing of each mineral (which he finds to be considerable), viscosity, 

 capillarity, the presence of water and mineralising agents, and the 

 absorption of adjacent rocks. 



To choose a simple example : minerals such as zircon, corundum, 

 and titanite separate for the most part early, because they are less 

 soluble. On the other hand, magnetite is one of the more soluble 

 minerals, and yet it is one of the first to separate ; the same is to 

 a certain extent true of augite, but not always. It is possible that 



