ANNIVERSIKY ADDRESS OF THE PRESIDENT. I77 



they subsided with the rest of the ground around them, and that 

 usually they were buried under overlying conformable sedimentary 

 deposits. Thus cones of ashes and lava which may have been 

 several thousand feet high completely disappeared. 



11. If we consider the nature and composition of the volcanic 

 materials so abundantly distributed in the British Isles, we may 

 deduce some conclusions of much interest in relation to the changes 

 which take place in the molten magma whence these materials pro- 

 ceed. I have shown that it is sometimes possible to observe a notable 

 difference in mineralogical and chemical composition between two 

 adjacent parts of the same mass of lava. In the Blackburn picrite, 

 for example, there has been a separation of the heavy basic con- 

 stituents, which have largely settled below, while the lighter felspar 

 has mainly come to the top. 



What can be shown to have taken place on a small scale in one 

 erupted sheet must always have been tending to occur in the main 

 mass of the magma itself, when the conditions for separation were 

 favourable. If we reflect upon the sequence which T have described 

 as so generally to be found in the petrographical characters of the 

 successively erupted materials, we may, it seems to me, detect the 

 proofs of a remarkable series of changes in the composition of the 

 magma during a volcanic period. 



With the important exception of the Snowdonian region and 

 possibly others, we find that the earlier eruptions of each period were 

 generally most basic, and that the later intrusions were most acid. 

 Thus the diabase-lavas and tuifs at the base of the Cambrian series 

 of St. David's are pierced by quartz-porphyry veins. The porphyrites 

 of the Lower Old Eed Sandstone were succeeded by bosses, sills, and 

 dykes of granite, felsite, and lamprophyre. The eruptions of the 

 Carboniferous plateaux began with extremely basic lavas, and ended 

 with thoroughly acid felsites and quartz-porphyries. The basalts of 

 the great lava-fields of the Tertiary period are pierced by masses of 

 granophyre and even granite. 



There has evidently been a progressive diminution in the quantity 

 of bases and a corresponding increase in the amount of acid in the 

 lavas erupted during the lapse of one volcanic period. This sequence 

 is so well-marked and so common that it cannot be merely accidental. 

 The acid and basic rocks, occurring as they do at each volcanic 

 centre in the same relation to each other, are obviously parts of one 

 connected series of eruptions. We seem to see in this sequence an 

 indication of what was taking place within the magma. Apart 



