188 Reports and Proceedings—Geological Society of London. 
Professor W. W. Watts, Sc.D., M.Se., F.R.S.; Henry Woods, M.A.; Arthur 
Smith Woodward, LL.D., F.R.S., F.1.8. ; Horace Bolingbroke Woodward, F.R.S.; 
and George William Young. 
Orricers :—President: Professor W. W. Watts, Sc.D., M.Sc., F.R.S. Vice- 
Presidents : Charles William Andrews, B.A., D.Sc., F.R.S.; Alfred Harker, M.A., 
F.R.S.; Horace W. Monckton, Treas.L.8.; Professor W. J. Sollas, LL.D., Se.D., 
F.R.S. Secretaries: Professor Edmund J. Garwood, M.A.; A. Smith Woodward, 
LL.D., F.R.S., F.L.8. Foreign Secretary: Sir Archibald Geikie, K.C.B., D.C.I.., 
LL.D., Sc.D., Pres.R.S. Zreaswrer : Aubrey Strahan, 8c.D., F.R.S. 
II. February 23, 1910.—Professor W. W. Watts, Sc.D., M.Sc., 
F.R.S., President, in the Chair. 
The following communication was read :— 
‘‘Metamorphism around the Ross of Mull Granite.” ! By Thomas 
Owen Bosworth, B.A., B.Sc., F.G.S. 
The Ross of Mull granite is a coarsely crystalline plutonic mass, 
forming the western portion of the Ross of Mull and extending over 
some 20 square miles. 
The intrusion is conspicuously later than the Moine rocks, and is 
regarded as one of the ‘newer granites’. The rock shows very little 
evidence of faulting or movement of any kind, and is traversed by 
sheets of mica-trap. The eastern boundary of the granite is a very 
intricate line of junction with typical Moine Schists and Gneisses, into 
which it has been intruded. Injection-breccias occur along the margin, 
where the granite is crowded with schist-inclusions. 
The changes in the pelitic schists are of two kinds, and are con- 
sidered under separate headings (a) and (8) below. 
(a) Impregnation.—The schists have been impregnated with the 
granite in a very intimate manner—(1) along irregular cracks, 
(2) along bedding-planes, (8) along strain-slip, and (4) along 
foliation. 
Variously banded rocks have been thus produced, which suggest 
how readily these processes, carried out on a large scale, would 
convert pelitic sediments from the state of schists into crystalline 
igneous gneisses. 
(6) Thermal Metamorphism.—In some places the pelitic gneiss in 
contact with the granite, and commonly the masses included in the 
granite, have been very highly altered. The new minerals formed are 
sillimanite, andalusite, cordierite, and green spinel; and these are 
present in such amount that their formation must have been accom- 
panied by much recrystallization among the quartz, felspar, and 
mica also. : 
Sillimanite is the most abundant new mineral, and occurs not only 
as fibrolite throughout the rock, but also in larger crystals which are 
often grouped together in prismatic aggregates. These aggregates 
weather out as conspicuous knobs, measuring about an inch across. 
Under the microscope, the sillimanite is seen to enclose large 
numbers of grains of green spinel. The~cross-sections of sillimanite 
are diamond-shaped, and show a pinacoidal cleavage; their colour 
1 Communicated by permission of the Director of H.M. Geological Survey. 
