THE METAMORPHIC ROCKS OF ADELIB LAND. 8T1LLWELL. 97 



pyroxene gneisses by secondary metamorphic processes. It is interesting to note that 

 this type of change of pyroxene to hornblende is just that which Grubenmann would 

 describe in transition from the kata zone conditions to the meso zone conditions. 



The phenomena illustrated on Plates VI., XII., XIV., XVIII. of this British Memoir 

 are exactly paralleled in the Cape Denison area. Plate VI. would there be described 

 as a stage in the metamorphic differentiation of the constituents of a primary basic 

 rock. In Plate XII. the hornblende differentiation is more advanced, and an imperfect 

 banded arrangement appears. Plate XIV. may have originated where a basic dyke 

 has run out into parallel threads, or it may be again metamorphic differentiation. A 

 dyke fragment with smaller pieces detached from it, together with a certain amount 

 of migration, might give rise to an appearance similar to that on Plate XVIII. 



After the publication of the preceding memoir in 1907 the Geological Survey of 

 Great Britain has produced a series of memoirs dealing with the North- West Highlands. 

 These memoirs provide the explanation of the published quarter sheets, but, like the 

 1907 memoir, are largely a mass of field data that await the co-ordinating process of 

 some worker. 



In the " Geology of the Seaboard of Mid Argyll " (Memoir No. 36, 1909) I have 

 noted an interesting (p. 6} description of phacoids of epidiorite in pebbly limestone 

 matrix. These phacoids are recognised as fragments torn off epidiorite dykes during 

 crushing, and the limestone is considered to have played the part of a plastic matrix. 

 This is another illustration of what may happen to a dyke sheet submitted to strong 

 stress. 



In the geology of Glenelg, Lochalsh, and south-east part of Skye (Memoir No. 71 

 1910), there is an excellent illustration (Plate V.) of a large knot of foliated basic rock 

 in the Lewisian gneiss at Rudha Gaol. The general appearance of this knot could be 

 matched among the disrupted dykes at Cape Denison. At Rudha Gaol the basic 

 inclusions have varying composition, indicating metamorphic diffusion or metamorphic 

 differentiation, or both. Another illustration (Plate VIII.) of the same memoir provides 

 a good example of metamorphic differentiation where a basic lenticle is illustrated in 

 . thinly-banded hornblendic gneiss. 



In the geology of the Fannich Mountains and the country around Loch Maree and 

 Strath Bromm (Memoir No. 92, 1913), we have again reference to the exposures which 

 have been illustrated in the large 1907 Memoir, and which we have considered to provide 

 examples of metamorphic diffusion and metamorphic differentiation. The explanations 

 given in 1907 are adhered to, and Plate VI. is still the picture of a magma of intermediate 

 composition from which the more distinctly basic and acid rocks are in the process of 

 formation. Another view is stated which regards the bulk of the rock as a mixture 

 rock formed by the mingling, probably while in a semi-fluid state, of basic and acid 

 portions. This latter view is a step nearer to the recognition of metamorphic diffusion 

 phenomena. There is also recognition (p. 23) of transitions by insensible gradations 



SariM A, VoL Hi., Part 1 G 



