E. B. Bailey 8f G. W. Grahham — Plagiodase Fehpars. 253 



becomes apparent that the albitization canuot have been accomplished 

 by mere weathering. The fact is there is no available source of soda 

 in the rocks as we find them to-day ; for, while they are practically 

 holocrystalline, their felspars are the only soda-bearing minerals 

 present, and it will be remembered that of these it is the more basic 

 members which are first transformed. 



But it is certain that the process of albitization does not antedate 

 the crystallization period of the ground-mass felspars ; where the core 

 of a phenocryst has been replaced the boundary between core and rim 

 is quite irregular ; when examined closely the albite extends along 

 cracks out into the rim, and it is obvious that the latter had formed 

 before the alteration took place. Then, too, the ground-mass felspars 

 are occasionally themselves albitized. We are thus led to suppose that 

 the albitization was performed shortly after the consolidation of the 

 ground-mass, and that the agency at work consisted of residual 

 solutions. We may advance the hypothesis that in certain A-olcanic 

 centres, e.g. Arthur's Seat, some portions of the magma were discharged 

 exceptionally rich in carbon dioxide (or some other unknown 

 constituent) ; that during crystallization of the lava an unusual 

 proportion of soda was thus retained in solution; and that the residual 

 liquors then began to react with the minerals which had crystallized. 

 Olivine was converted into serpentine and chlorite, while the most 

 basic felspars were replaced by albite. The process may be crudely 

 expressed by saying that the lava at this stage was stewing in 

 a concentrated solution of sodititn carbonate. 



Many specimens from Arthur's Seat, where albitization has 

 proceeded unusually far, offer additional support to the interpretation 

 given above. It is common to find the vesicles of these rocks lined 

 with beautiful little, inwardly radiating crystals of albite, forming 

 a definite outer layer. An inner layer of chlorite, either pennine 

 (11,991)^ or delessite (12,027), follows the albite, while the centres 

 of the vesicles are occupied by calcite. It is also a feature of these 

 vesicles that they frequently communicate one with another by means 

 of branching cracks, which are usually entirely occupied by albite 

 (PI. X, Fig. 2). The cracks traverse phenocrysts and ground-mass 

 alike, showing that the lava had consolidated at the time of their 

 formation. The albite occupying them is generally crystallized in 

 lath-shaped form, as in the ocellar structures of camptonites, but 

 a marginal layer is often deposited in optical continuity with the 

 albitized felspars which have been broken across. Thus, in the lams 

 which have been most completehj albitized, there teas still a surplus left to 

 line the vesicles and fill the cracks formed in connection with the 

 same. To claim vesicle minerals as juvenile products is nothing new,* 

 and certainly it is difficult to regard the albite of the Arthur's Seat 

 examples in any other light. Chlorite, which we may suppose 

 originated from the decomposition of the original olivine, evidently 

 crystallized out of solution after the albite, and last of all came calcite. 



^ The numbers given in parentheses refer to slides in the Geological Survey 

 collection. 



* Cf. A. Harker, "The Tertiary Igneous Rocks of Skye": Mem. Geol. Survey, 

 1904, chap. iv. 



