FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 7 



greenish clay-like substance which, when crushed and examined under the micro- 

 scope, appears to consist of a mixture of prehnite and yellowish-brown isotropic 

 chloritic material. 



Garnet. — This mineral is an unusual associate of zeolites, but on An Gearna it 

 is of fairly frequent occurrence. Its presence has already been noted by Professor 

 Judd,* who mentions it as occurring with epidote in the volcanic agglomerates of 

 the Hebrides. Mr Currie, in the paper referred to above, notes its absence on Maol 

 nan Damh, but there can be no doubt that Judd in his description had in mind the 

 locality under consideration, although the rocks in which I have detected the mineral 

 show none of the characters of volcanic agglomerates. The garnet varies in colour 

 from pale yellow, or colourless, to deep brown, a beautiful wine-yellow tint being 



Fig. 3. — Crystal of garnet from An Gearna, Mull. 



commonest. It occurs usually in crystals — occasionally *5 cm. in diameter — 

 studding the scolecite near the junction of that mineral with the underlying layer of 

 epidote, prehnite, etc., although sometimes it is found well in the centre of the zeolite. 

 The common form is the rhombic dodecahedron the faces of which are usually 

 curved and irregular, but the form shown in fig. 3 has also been noted. In 

 crystals of this shape it is of interest that the icositetrahedral faces show the unusual 

 type of striation, parallel to the intersection of the icositetrahedron and the 

 dodecahedron, noted by Professor Shand f in his description of the garnets from 

 Corsie Hill Quarry. 



Besides occurring in direct association with scolecite, garnet has also been found 

 with prehnite and albite, forming a layer between the two, and, rarely, imbedded 

 in calcite. Much of the massive white prehnite found on the hill is spotted with 

 garnet, and it is noteworthy that whenever the latter mineral occurs with scolecite, 

 prehnite occurs in direct association with the two. 



* J. W. Jodd, "On the Secondary Rocks of Scotland," Quart. Journ. Geol. Soc, vol. xxx, 1874, p. 241. 

 t S. J. Shand, Proc. Perth. Soc. Nat. Sci, 1907, vol. iv, p. 210. 



