part 3] XENOLITHIC MII^'OR INTRUSIONS IN MFLL. 229 



6. On Certain Xenolithic Tertiary Minor Intrusions in 

 the Island o/'Mull (Argyllshire). Bj Herbert Henry 

 Thomas, M.A., Sc.D., V.P.Gr.S. ; with Chemical Analyses 

 by Ernest GtEorge Radley, F.C.S. (Read May 19th, 

 1920.) 



[Plates VI-X.] 



Contents. 



Page 

 I. Introduction 229 



II. The Intrusions : General Description, Distribution, and 



Petrography 231 



III. The Cognate Xenoliths (Enclaves Homoeogenes) 238 



IV. The Accidental Xenoliths (Enclaves Enallogenes) 239 



(a) The Siliceous Xenoliths 239 



(6) The Aluminous Xenoliths 240 



(i) General Description, 

 (ii) The Minerals of the Xenoliths : their Mode of 



Formation and Mutual Relations, 

 (iii) Probable Nature of the Unaltered Xenolithic 



Material, 

 (iv) Relation of the Xenoliths to the Enclosing 



Rock. 

 (v) The Conditions of Metamorphism. 



V. Summary and Conclusions 255 



I. Introduction. 



In the years 1910 & 1911, the officers of the Scottish branch of 

 H.M. Geological Survey, while engaged on a survey of that portion 

 of the Island of Mull which lies south of Loch Scridain, were 

 impressed by the great number of minor intrusions of a tholeiitic 

 and andesitic character which penetrate the western part of the 

 Tertiary lava-field. These rocks were described as being, for the 

 greater part, of one type : namely, olivine-free dolerites or tholeiites, 

 of which many have pitchstone centres, and in which a sheatli- 

 and-core structure is frequently developed.^ 



Mr, E. M. Anderson noted the occurrence within them of 

 xenoliths of apparently fused or baked sandstone, and also recorded 

 certain nodular masses of bytownite and some ferromagnesian 

 mineral which he compared with similar masses, collected by the 

 late C. T. Clough and described by Dr. J. S. Flett, from a tholeiitic 

 intrusion at Traigh Bhan an Sgoir^ (Traigh Bhan na Sgurra, 

 Locality 11, fig. 1, p. 230). 



^ B. Lightfoot, ' Summary of Progress for 1910' Mem. Geol. Surv. 1911, 

 p. 30 ; E. M. Anderson, ' Summary of Progress for 1911 ' Mem. Geol. Surv. 

 1912, p. 34. 



^ ' The Geology of Colonsay & Oronsay, with part of the Ross of Mull ' 

 Mem. Geol. Surv. 1911, p. 92. 



Q.J. a. S. No. 311. R 



