FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 29 



basic plagioclases appear to be unstable, their representatives being albite and a lime- 

 alumina silicate, such as prehnite, containing more or less combined water. 



Hornblende is a common mineral in the metamorphosed amygdales, and has been 

 formed by the alteration of the chlorite. It varies considerably in colour, being some- 

 times deep green and strongly pleochroic and sometimes very pale yellowish-green 

 [9, 55, 59, 101]. It occurs in prehnite, epidote, and scolecite, and is always fibrous 

 in habit. The change from chlorite to hornblende or biotite is quite a well-known 

 effect of contact-metamorphism,* and, as lime is plentiful in the amygdales, and 

 alkalies relatively scarce, hornblende has always been formed in preference to 

 the mica. 



These changes, which can be followed in detail in the less altered amygdales, 

 become much more intense as the granophyre is approached. The solid primary 

 prehnite, as well as the fibrous secondary material derived from the zeolites, is con- 

 verted to garnet and epidote. The garnet varies considerably in appearance. It 

 may be white to dark brown in colour and occurs either as idiomorphic crystals in 

 the prehnite or as large irregular masses traversing and enclosing it. The mineral is 

 frequently isotropic, but optical anomalies are also common ; in some cases large 

 rounded crystals consist of a brown, isotropic centre and a colourless, birefringent 

 rim. Inclusions of epidote and prehnite are fairly common [26, 32]. 



The epidote is colourless, and crystals show pronounced zonal structure and re- 

 markable variation in birefringence; occasionally sections show indigo-blue polarisa- 

 tion colours with positive optical sign, resembling in this respect zoisite rather' than 

 epidote. Inclusions of prehnite are sometimes present [25]. 



Sections through the amygdales collected from the screes lying nearest the grano- 

 phyre margin on the north-east slope of Beinn Fhada show the most intense type of 

 alteration noted. The zeolites have completely vanished, and their place is taken by 

 a dull turbid substance which preserves their radiate structure. It is isotropic, and 

 under high powers is seen to consist of minute yellowish grains of garnet. Any 

 prehnite which has survived is studded with small hexagonal sections of garnet 

 which is also replacing the epidote. The latter mineral occurs in crystals part of 

 which consists of garnet, or it forms a corroded patch surrounded by garnet which 

 sends off growths into the epidote [31, 40, 41]. 



Sphene is frequently present in the slides and forms grains and rounded crystals 

 enclosed in the epidote or in the adjacent prehnite, whilst the chlorite has gone to 

 hornblende, which sometimes forms compact green patches showing good cleavage 

 and strong pleochroism [46]. 



III. Summary and Conclusions. 

 It is clear from a study of the metamorphosed amygdales that they were filled 

 prior to the intrusion of the granophyre and that they participated in the meta- 



* A. Marker, loc. cit., p. 50. 



