B. K. N. Wyllie & A. Scott — Plutonics of Garabal Hill. 503 



The significant phenomena are these : ( 1 ) cored hornblende crystals,^ 

 showing either actual diallage, or the diallage schiller structures, in 

 their centres; (2) partly pleochroic diallage crystals, retaining the 

 pyroxene cleavage and extinction ; (3) amphibolized veins meandering 

 through diallage crystals. 



The last case especiallycan hardly be explained on the 'simultaneous' 

 view, whereas the ' secondary ' theory explains all equally well. 

 Moreover, it may be doubted if a mere equilibrium change during 

 the original cooling suffices to account for the formation of a strongly 

 coloured and strongly pleochroic amphibole from a practically 

 colourless pyroxene. Amphiboles in general are reputed to contain 

 hydroxyl, whereas pyroxenes do not. It is possible that the 

 introduction of hydroxyl into a molecule gives it different optical 

 properties : for instance, amphiboles are pleochroic ; pyroxenes, as 

 a rule, are not. But the case we have at present seems to demand 

 still more than this ; it is difficult to associate the marked brown of this 

 hornblende with anything but a marked increase in the ferric-oxide 

 content. We suggest, therefore, that hydroxylation and oxidation 

 have been effected by heated vapours, and general reheating, from 

 the subsequent diorite magma. 



We offer such chemical evidence as we possess in support of this 

 idea. A specimen of davainite was analysed, but as it was more 

 felspathic than tlie typical rock we have recalculated the analysis to 

 give the proportions of the femic constituents only : — 



I. 



la. 



II. 

 III. 

 IV. 



V. 



Davainite. 



,, recalculated. 

 Wehrlite (somewhat serpentinized), L. Garabal, anal. J. H. Player.^ 



J. -V /-Koswinsky Kamen, North Ural. 



Peridotite, Dunan Liath, Boss-shire, anal. W. Pollard.^ 



1 Teall & Dakyns, loc. cit., p. 115. 



2 Geologij of Ben Wyvis, etc. (Mem. Geol. Surv. Scotland), 1912, p. 127. 



