Vol. 57.] ANNIVERSARY ADDRESS OF THE PRESIDENT. IxXXlii 



the district. Now, a microscopic examination of the andesites proved 

 that the phenocrysts taken together must have the composition of a 

 basic rock, for they were composed of labradorite, augite, and 

 hypersthene, and therefore the glassy base present in some of the 

 andesites must be allied to quartz-felsite in composition. The 

 sequence established by Mr. Clough could therefore be explained by 

 the assumption that the quartz-felsite magma represented the mother- 

 liquor of the andesitic magma, after the phenocrysts had separated 

 out. Thus, if crystallization had progressed in the plutonic mass to 

 the stage represented by the phenocrysts of the lava, or a little 

 further, and the mother-liquor had then been squeezed out as one 

 squeezes water out of a sponge, or separated in any other way, and 

 forced upwards into cracks in the overlying series of andesitic lava- 

 flows, the question above referred to could be satisfactorily answered. 

 I was fortunately able to test the theory quantitatively, for 

 Mr. Waller had already analysed one of the quartz-felsites and 

 Dr. Petersen had published analyses of the glassy base of one of the 

 andesites and of the devitrified base of another. On comparing the 

 mean of the two analyses of the base with the analysis of the quartz- 

 felsite, it was found that of the eight constituents, six differed by 

 less than -4 per cent., silica differed by 2*2, and soda by 1-46.^ 



Differentiation dependent on crystallization is a fact which can- 

 not be denied ; for the igneous magma, except when it cools as a 



^ As the figures were not placed side by side in the original paper (Geol. 

 Mag. 1885, p. 106), I so place them now : — 



I. II. III. IV. Diff. 



SiOa 66-25 65-16 65-70 679 -|-2-20 



AlA 13-59 17-49 15'54 157 +0-16 



FeA 311 3-01 306 30 -006 



OaO 2-75 0-84 179 1-4 -0-39 



MgO 0-28 2-34 1-31 1-5 +019 



K,0 4-95 5-54 5-24 56 +0-30 



Na^O 2-25 3*68 296 1*5 -1-46 



Loss 5-89 1-76 3-82 37 -012 



99-07 99-82 99-42 100-3 



I = Grlassy base of hypersthene-andesite from Fairhaugli, Usway Earn, 

 Cheviots. (Ebert.) 



II = Devitrified base of andesite, 2 miles up Allerhope Burn. (Wulf.) 



III = Mean of the two analyses. 



IV = Quartz-felsite from dyke on the Coquet, | mile above Shillmoor Farm, 

 Cheviots. (Waller.) 



