FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 23 



similar petrographical character are common among the plateau-basalts, and their 

 felspars have not been albitised. It must, however, be conceded that albitisation is 

 of universal occurrence in the rocks around the plutonic centre, although, in this 

 area, it is only in the localities mentioned that the peculiar association of minerals 

 in the vesicles has been detected. The cause and date of this general albitisation 

 can be settled only by an examination of the rocks over the whole area, but the 

 frequent association of the vesicle-minerals with those of igneous origin in the 

 basalt of Maol nan Damh indicates that the change took place, in this case, during 

 the last stages of cooling of the lava itself. 



The albitised felspars almost invariably contain veins and patches of chlorite, 

 which, as noted above, sometimes replaces entirely the original plagioclase. There 

 does not seem to have been much introduction of soda, and the process was probably 

 carried out by solutions containing chlorite and capable of dissolving the anorthite 

 of the plagioclase. These solutions also partially chloritised the augite, and subse- 

 quently deposited their loads in the vesicles in the form of chlorite, prehnite, epidote, 

 and scolecite, any surplus soda going to form albite,' and, only very exceptionally, a 

 zeolite containing soda. This rarity of soda-zeolites is an important point. 



From the amount of secondary albite in the vesicles it is clear that soda was 

 present in considerable quantity in the solutions, yet in nearly every case it has been 

 deposited as albite — a fact which indicates strongly that the vesicle- minerals were 

 deposited at a fairly high temperature. 



Doelter, in a suggestive paper,* has pointed out that, under laboratory conditions, 

 a mixture of soda, alumina, silica, and water deposits analcite between temperatures 

 of 190° and 420° C. ; at lower temperatures natrolite is formed, and at higher 

 temperatures either nepheline or albite. The formation of analcite seems to depend 

 largely upon the temperature and to a very slight extent upon the relative pro- 

 portions of the constituents, for it has been formed from solutions of very different 

 compositions.*)* The conditions influencing the formation of albite are not so clearly 

 understood, but, as shown by the work of Baur,J and Friedel and Sarasin,§ a 

 temperature of 500° C. appears to be necessary. 



As regards the sequence of events in the vesicles it is certain that their infilling 

 was a continuous process, and the succession of minerals is what we would expect 

 from deposition under conditions of falling temperature. Chlorite seems to have 

 been deposited during all the stages, a fact remarked upon by Fenner || in his account 

 of the Watchung basalt, but only very exceptionally does the vesicle-albite enclose it. 

 It appears to have been formed in large quantities during, or slightly previous to, 

 the deposition of epidote, which frequently encloses it and never shows sharp junctions 



* C. Doelter, Tsclxermak's Min, Pet. Mittheil., 1906, vol. xxv,p. 97. 



t hoc. cit, p. 102. 



X E. Baur, Zeifs.f. physikal. Chimie, 1903, vol. xlii, p. 570. 



§ C. Friedel and E. Sarasin, Compt. rend., 1883, vol. xcvii, p. 290. 



|| C. N. Fenner, he. cit., p. 174. 



