FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 31 



ance which the garnet presents in the vesicles where it does occur, it is quite possible 

 that it may have been present more generally during the first phase of deposition 

 and that it became obliterated during the succeeding stages. 



A CKNO WLEDGMENTS. 



I take this opportunity of expressing my best thanks to Dr J. S. Flett and 

 Mr E. B. Bailey, B.A., of H.M. Geological Survey, for the interest which they have 

 taken in the investigation and for the many valuable suggestions which they have 

 made during the course of the work. To Dr Robebt Campbell, of the University, 

 and Mr F. D. Miles, B.Sc, of the Heriot-Watt College, I am greatly indebted for 

 help in the photographic part of the work. My gratitude is also due to the Carnegie 

 Trust for defraying the cost of illustrating this paper. 



EXPLANATION OF PLATES. 

 Plate I. 



Fig. 1. Ophitic Basalt [107] with Vesicles, magnified 27 diameters. — The light area to the right is a 

 large vesicle containing prehnite and chlorite ; at the bottom an acicular augite projects into the pi'ehnite. 

 The small vesicle to the left is filled with prehnite, enclosing a little chlorite ; the dark crystals projecting 

 into the prehnite are augite. Maol nan Damh, Ben More, Mull. 



Fig. 2. Ophitic Basalt [70] with Vesicle, magnified 27 diameters. — To the right at the bottom is the 

 rock, much decomposed, with numerous grains of magnetite. The large crystal projecting into the vesicle is 

 purple augite pierced by laths of albite. To the left it shows a fracture filled with white scolecite, which 

 occupies the rest of the cavity. The darker patches in the scolecite are chlorite. Just above the fracture 

 in the augite is a crystal consisting of purple augite (dark) and pale yellow epidote (light). Maol nan Damh, 

 Ben More, Mull. 



Fig. 3. Ophitic Basalt [23] with Vesicle, magnified 1 2 diameters. — The rock, at the bottom, is succeeded 

 by a pegmatitic layer consisting of lath-shaped crystals of purple augite (white), patches of chlorite (dark), 

 and albite. At the junction with the rock the augite projects radially to the wall, but towards the vesicle 

 they lie tangentially. This layer is succeeded by a zone of chlorite and epidote, above which lies albite 

 more or less turbid. Maol nan Damh, Ben More, Mull. 



Fig. 4. Ophitic Basalt [110] with Pegmatitic Layer underlying Vesicle, magnified 27 diameters. — The 

 layer on the right consists of lath-shaped crystals of augite, magnetite, chlorite, and albite. The clear, 

 irregularly shaped patches are albite enclosing a few crystals of augite. Maol nan Damh, Ben More, Mull. 



Fig. 5. Ophitic Basalt [111] with Vesicle, magnified 27 diameters. — The rock, to the left, is a dense 

 aggregate of augite, chlorite, and magnetite. The pegmatitic layer is well developed and consists of laths 

 of augite, albite, magnetite, and chlorite. The white mineral in the vesicle is prehnite into which some of 

 the underlying augite has wandered. Maol nan Damh, Ben More, Mull. 



Fig. 6. Ophitic Basalt [111] with Vesicle, magnified 27 diameters. — The dark patch at the foot 

 is the rock, which here consists of augite and chlorite. The turbid patches with black outlines which 

 stretch on the left from the rock across the vesicle are albite which is being replaced and corroded by 

 prehnite. Just above the middle patch of albite are two lath-shaped crystals of purple augite. The 

 remaining clear and turbid areas consist of prehnite which, when it is turbid, has replaced albite. To the 

 left of the central patch of albite there is a clear area consisting of prehnite surrounded by a turbid zone 

 consisting of rectangular phantom crystals of albite replaced by prehnite optically continuous with the clear 

 central portion. Maol nan Damh, Ben More, Mull. 



