256 E. B. Bailey 8f G. W. Grahham — Plagioclase Felspars. 



arrangement, while the quartz is interstitial and sometimes micro- 

 pegmatitic. Calcite occurs not infrequently, filling the centres of 

 drusy cavities lined by quartz crystals. Analysis proves the absence 

 of orthoclase, so that the nature of the felspar is sufficiently indicated 

 by the fact that its highest refractive index is well below the lowest 

 of quartz. 



I^ow, one cannot but recognize that these segregation veins present 

 a close analogy with the albite veins connecting vesicle to vesicle in 

 the lavas of Arthur's Seat, while at the same time theii* juvenile 

 origin is so obvious that it has been taken for granted by every writer 

 on the subject. They are so numerous, too, that it is clear that, during 

 the period of their crystallization, the heated interior portions of the 

 sills must have been permeated through and through by soda-rich 

 solutions. It is very difficult to avoid the conclusion that it was under 

 these propitious conditions that the albitization of the originally basic 

 felspars of the consolidated rock was accomplished, together with the 

 production of much of the accompanying white mica. 



It will thus be seen that the evidence derived from a consideration 

 of the Permo-Carboniferous intrusions lends some support to the view 

 already arrived at in regard to the processes involved in albitization as 

 exemplified in the Carboniferous lavas. In conclusion, the analogy 

 between this change and another well-known type of decomposition, 

 prevalent in certain igneous rocks of Scotland, may perhaps be alluded 

 to. Dr. Flett has found, in the Midlothian teschenites, a replacement 

 of basic soda-lime felspar by analcite, similar to the replacement by 

 albite in the quartz dolerites, and he regards the change as juvenile or 

 pneumatolytic in character. That analcitization and albitization may 

 be very closely comparable phenomena will readily be granted when it 

 is remembered that, chemically, 



analcite + silica = albite + water. 



EXPLANATION OF PLATES X AND XL 



MiCROPHOTOGRAPHS BY Mr. T. C. HaLL. 



Fig. 1. X 29 dia. Crossed nicols. Basic plagioclase, phenocryst all but com- 

 pletelj- replaced by secondary albite. A ragged patch of the original 

 felspar shows clear white near the top of the slide. (13,119.) 1400 feet 

 S. 30° W.. from cairn on I. airs, above Lennoxtown, Campsie Fells. 



Fig. 2. x 43 dia. Crossed nicols. Zoned plagioclase phenocryst. The moi-e 

 basic poi'tions have been preferentially albitized. The centre consists almost 

 wholly of secondary albite, showing white ; and an alternating basic zone, 

 showing dark near the top corner, is being preferentially replaced by patches of 

 albite. (13,082.) Crichton's Cairn hill-top above Lennoxtown. 



Fig. 3. x 22 dia. Centre of felspar phenocr)'st entii'ely replaced by cloudy 

 secondary albite. The rim is plagioclase (not orthoclase), more basic than 

 albite, but doubtless less basic than the original felsjiar of the central portion 

 now replaced. (12,280.) 100 yards north-east of north corner of Aucheneden 

 Plantation, Kilpatrick Hills. 



Fig. 4. x 22 dia. Crossed nicols. Albitized basalt with albite vein. A small 

 round vesicle, filled with albite, is seen just below the vein and communicating 

 with it. These veins appear merely to connect up neighbouring vesicles with 

 one another and not to traverse the rock for any distance. (12,027.) Calton 

 Hill, path opposite west wing of High School, Arthur's Seat volcano. 



