﻿37 6 MR. T. 0. B0SW0RTH ON METAMORPHISM [Aug. I9IO, 



15. Metamorphism around the Boss oe Mull Granite. 1 By Thomas 

 Owen Bosworth, B.A., B.Sc, F.G.S. (Bead February 23rd, 

 1910.) 



Contents. 



Page 

 I. The Granite and the Moine Gneisses 376 



II. Impregnation of the Pelitic Gneisses with Granite 380 



III. Contact-Metamorphism of the Pelitic Gneisses 385 



IV. Kyanite and Tourmaline-Rocks ascribed to the Regional 



Metamorphism 391 



I. The Granite and the Moine Gneisses. 

 (a) The Granite. 



The western end of the Boss of Mull, which stretches out into the 

 Atlantic, is a granite mass occupying some 20 square miles of land. 

 On the north, west, and south this granite is exposed to the ocean 

 waves, but along its eastern border it is seen in contact 2 with a 

 series of typical Moine Schists, which form the next portion of this 

 peninsula passing landwards. 



The Boss of Mull granite is a muscovite-biotite-granite of coarse 

 texture intruded into the Moine Schists and Gneisses. At the 

 south-western corner of the Boss, opposite the island of Erraid, it 

 gives place to a fairly coarse diorite, which also forms the Eilean 

 a' Chalmain. 3 The granite was intruded probably before the diorite 

 was cool, and the two rocks are there so intermixed that the drawing 

 of any boundary-line between them is a purely arbitrary matter. 

 Taken together, the facts summarized below lead to the conclusion 

 that this granite is one of the ' Newer Granites,' as was formerly 

 inferred by Prof. Judd 4 : — 



(a) It resembles the newer granites in composition and association with 



diorite. 



(b) It is unaffected by shearing. 



(c) It is conspicuously later than the foliation and strain-slipping of the Moine 



Schists into which it has been intruded, producing contact-alteration. 



(d) It is associated with sheets of mica-trap and vogesite, by which it is 



traversed. In one case, 3 an intrusive sheet 2^ feet thick which 

 cuts the Moine rocks is itself penetrated by strings and tongues of 

 granite from an offshoot of the granite mass. This sheet is slightly 

 foliated at the surfaces, but is probably younger than the normal 

 epidiorites of the district. 



1 Communicated by permission of the Director of H.M. Geological Survey. 



2 J. G. Goodchild, Geol. Mag. 1892, p. 447. 



3 Summary of Progress of Geol. Surv. for 1907, p. 66. 



4 Quart. Journ. Geol. Soc. vol. xxx (1874) pp. 244 & 290 ; see also Summary 

 of Progress of Geol. Surv. for 1907, p. 66, & ibid, for 1908, p. 55. 



5 A quarter of a mile south of Rudha na Traighe-maoraich, on the shore of 

 Xoch na Lathaich. 



