20 ME. H. C. SAEGE>T OX LO^ER CAI{EO:NIFEEOrS [yoI. Ixxiii,. 



in a volatile state. Prof. R. A. Daly^ cites strong corroboration, 

 of his view that 



' the abundant soda of a spilite has been concentrated from an underlining- mass 

 of normal basaltic magma ' 



hy gaseous transfer of the albite molecule. Dr. F. W. Clarke ^ is 

 also of opinion that 



' the magmatic vapours must exert an important influence upon the process of 

 differentiation, for thej tend to accumulate in the upper part of a lava column 

 or reservoir and to modify its properties locally.' 



A further consideration, which api^ears to have an impoi-tant 

 bearing on the evolution of the spilites, is the effect of the physical 

 environment of a submarine flow. It is generally recognized that 

 the pressure of the superincumbent mass of water on the molten 

 rock. would effect the rapid formation of a thin crust on its surface, 

 with the twofold result that its internal heat would be retained 

 longer than in the case of a subaerial flow, and the release of the 

 volatile constituents would be im^^eded. 



The presence of hot alkaline solutions thus retained and circu- 

 lating among the earlier-formed minerals affords a ready explana- 

 tion, as has been noted by other observers, of the intense alteration 

 seen in the spilites. Post-volcanic or juvenile emanations have also 

 been invoked, but retained residual solutions, especially in the case 

 of outcrops far removed from any recognizable vent, seem quite 

 sufficient for the pm'pose. 



It has been already pointed out that the basalts, which were 

 erupted later than the spilites, are often beautifull}' fresh, although 

 they have, of com'se, been subjected to the same external influences 

 since ei-uption as the spilites. This affords further evidence that 

 the altei-ation of the spilites cannot be due to weathering. It 

 a])pears, rather, to be the flnal stage of consolidation. 



The infilling of the vesicles presents features which, it is sug- 

 gested, may be referred to the prolonged retention of a high 

 temperature in the lava after effusion. It has already been stated 

 that the vesicle-minerals are chlorite, albite, quartz, chalcedony, 

 calcite, and dolomite. Seeing that augite was one of the latest 

 minerals to consolidate, it is j^i'obable that the hydrated residue 

 contained augite-forming bases, and thus the earliest-formed 

 chlorite, the lining of the vesicles, which differs widely in character 

 from the helminth and delessite formed at later stages, may, 

 perhaps, be regarded as a primary mineral. This seems to be the 

 view adA'ocated by Mr. James Strachan.'^ 



The amygdaloidal albite, too, may also be regarded as a primary 



"' ' Igneovis E-ocks & their Origin ' 1914, p. 339. 

 2 • The Data of Geochemistry ' 3rd ed. (1916) p. 311. 



■^ ' The Carnmoney Chalcedony -. its Occurrence & Origin ' Proc. Belfast 

 Nat. Field Club, vol. ii (1906). 



