172 DR. C. E. TILLEY ON THE PETROLOGY OF [vol. Ixxix, 



8. The Petrology of the Metamorphosed Rocks of the Start 

 Area (South Devon). By Cecil Edgar Tilley, Ph.D., 

 B.Sc, A.I.C... F.G.S. (Read June 28th, 1922.) 



[Plates X & XI.] 



Contents. 



Page 



I. Introduction 172 



II. Structure of the District 173 



III. The Mica- and Quartz-Mica-Schists 178 



(a) Mica-Schists of the Start-Portlemouth Area. 



(b) Mica- Schists of the Bolt Area. 



(c) Petrography. 



IV. The Green Schists 180 



(a) General Description. 



(b) Petrography. 



(i) Chlorite-Epidote-Albite-Schists. 

 (ii) Hornblende-Epidote-Albite-Schists. 

 (iii) Nodular Masses in the Green Schists. 



V. Schists of Composite Origin 188 



ia) Quartz-Muscovite- Chlorite- Albite-Schists. 



(b) Quartz-Muscovite-Chlorite-Albite-Garnet-Schists. 



VI. The Metamorphic Boundary 190 



VII. Nature of the Metamorphism 193 



VIII. Comparison with other Green Schist Areas 199 



(i) The Lizard Area. 



(ii) The ' Green Beds ' of the Scottish Highlands, 

 (iii) The ' Mona Complex ' of Anglesey, 

 (iv) The Green Schists of Western Norway. 



I. Introduction. 



The narrow strip of country forming the southernmost peninsula 

 of South Devon, and extending from Start Point westwards to 

 Bolt Tail, has heen the subject of repeated enquiry hy various 

 geologists since the first report on the geology of Devon hy 

 Sedgwick & Murchison. 



On the north this area is bounded hy rocks the Devonian age 

 of which is attested by definite organic remains. Whether the 

 rocks of the Start peninsula themselves are also of this age, or 

 whether an older group, has heen much discussed, and no small 

 part of the literature is devoted to this subject, apart from any 

 exhaustive study of the rocks themselves. Prof. T. Gr. Bonney x 

 appears to have been the first to apply microscopic petrographic 

 methods to the elucidation of the rocks themselves, and, following 

 him, Miss C. A. Raisin 3 has also contributed researches on the same 



1 Q. J. G. S. vol. xl (1884) pp. 1-26. 



2 Ibid. vol. xliii (1887) pp. 715-33. 



