FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 



21 



while the exceptional cases, we may sum up the main features of the occurrence as 

 follows : — 



1. The rock is an olivine basalt with the purple augite typical of most of the 

 lavas in Mull. 



2. The felspars are albitised and veined with chlorite, and the olivine and augite 

 also show alteration to that mineral. 



3. The magma was rich in gases at the time of its eruption, and hence the vesicles 

 are large and plentiful. 



4. The cavities were sometimes the seat of igneous crystallisation subsequent to 

 their formation, and, in the coarser portions of the rock, the vesicle-minerals have 

 grown upon a pegmatitic layer of augite, albite, magnetite, and chlorite ; crystals from 

 this layer are occasionally enclosed in the contents of the amygdales. 



5. With the exceptions of chlorite and albite, the vesicle-minerals have lime for 

 their principal base ; zeolites containing soda are exceedingly rare. 



6. There is evidence of a more or less definite sequence in the deposition of the 

 minerals, namely : — 



(a) A layer of augite, albite, magnetite, and chlorite. 



(b) Albite. 



(c) Epidote. 



(d) Prehnite. 



(e) Scolecite. 



(a) Albite. 



(b) Epidote. 



(c) Prehnite. 



(d) Thomson ite. 



Other sequences are : — 



(a) Garnet. («) Garnet. 



(b) Diopside. (6) Albite. 



(c) Epidote. (c) Epidote. 



(d) Chlorite. 



(e) Scolecite. 

 (/) Heulandite. 



7. Calcite is of rare occurrence, and hydrous oxides of iron and other typical oxi- 

 dation products of weathering are almost entirely absent. 



These facts suggest that the vesicle-minerals were deposited during the final 

 period of cooling of the rocks, which seems to have been a long one. During the 

 period of igneous crystallisation the normal pyrogenetic minerals, olivine, labradorite, 

 and augite, were formed, the augite crystallising last. Consequently the magma, in 

 the last stages of this phase, was rich in the augite-forming bases, and whilst the rock 

 was solid, but necessarily at a high temperature, these formed the large, acicular 

 augites occurring in some of the vesicles. When felspar occurs with them it is always 

 albite in which inclusions are usually abundant. It forms turbid, lath-shaped crystals 

 with much included chlorite, and also irregular patches much clearer and compara- 

 tively free from chlorite, but containing frequently grains and crystals of purple 



