FKOM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 11 



of the vein are arranged roughly parallel to the walls. A few cavities have been 

 found in which prehnite is specially abundant. The underlying layer consists 

 of chlorite speared with acicular augite with which albite is intimately associated ; 

 in such cases the augite is occasionally found enclosed in the immediately 

 overlying prehnite. 



Owing to the dark colour of the chlorite and the frequent occurrence of fibrous 

 epidote, it is difficult to detect these acicular augites in hand-specimen. They are 

 best seen on the weathered surface of the chlorite, which is pale, and they can be 

 readily identified by an examination of a little crushed material under the microscope, 

 when the purple colour, cleavage, and positive optical sign distinguish the mineral 

 at once from epidote. They are a prominent feature in sections cut through the 

 junction of the rock with the vesicle, and their significance will be discussed when 

 the origin of the zeolites comes to be considered. 



In addition to vesicles, the lavas also carry veins filled with albite in which 

 are, occasionally, clear crystals of quartz and rhombohedra of chabazite. Veins 

 filled with albite, solecite, and prehnite also occur, and there are other minerals 

 which can only be detected under the microscope, and which will be described later. 



Meanwhile it is interesting to compare the differences, obvious in hand-specimen, 

 between the minerals of Maol nan Damh and those from An Gearna. First, we may 

 note that the dark green chloritic layer underlying the zeolites is much sharper and 

 better developed on Maol nan Damh than on An Gearna, where its place is usually 

 taken by a confused zone of yellow epidote with which tufts of green hornblende 

 are sometimes associated. The second notable difference lies in the colour of the 

 epidote predominant at the two localities. At Maol nan Damh it is typically of 

 a deep, bottle-green shade, whilst at An Gearna it is usually pale yellow, brown, 

 or pink ; the last variety has not been found at Maol nan Damh. A third difference 

 lies in the abundance of garnet on An Gearna and its absence or extreme rarity at 

 Maol nan Damh. On one or two specimens collected by myself I have noted very 

 small crystals in microscopic section, whilst on another collected by Mr Anderson 

 it is visible to the unaided eye ; Mr Currie notes its absence in the material 

 collected by him. 



It is clear, therefore, that on An Gearna we are dealing with the somewhat 

 metamorphosed representatives of the vesicle-minerals of Maol nan Damh, and it 

 is interesting to trace this well-defined zone towards the large intrusive mass of 

 granophyre which forms the centre of Beinn Fhada, a ridge running parallel to 

 An Gearna at a distance to the north-west of about a mile. 



The Minerals op Beinn Fhada. 



On this hill, the cavities are neither so plentiful nor so large as those obtained 

 at the localities previously described, but good specimens can be obtained from the 



