442 ME. A. HAEKEE ON THE EETJPTIVE EOCKS Df THE 



31. On the Eetjptive Eoce:s in tlie NeigTibourTiood of Saen, Caeeitae- 

 voNSHiEE. By AiEEED Haekee, Esq., M.A., P.G.S., Fellow of 

 St. John's CoUege, Cambridge. (Eead May 9, 1888.) 



I. Introductory. 

 n. The Granite and Gneissic Granite. 



III. The Gabbro, Diorite, and Gneissic Diorite. 



IV. The Diabase. 



V. The Hornblende-Diabase. 

 VI. The Hornblende-Picrite. 

 VII. The Dolerite Dykes. 



I. Inteodtjctoey. 



The district to be treated lies to the west and south of the village 

 of Sarn *, near the south-western extremity of Caernarvonshire. The 

 eruptive rocks there exposed, excluding outlying patches, occupy an 

 area of irregular shape, which extends about 5| miles from north 

 to south, and has a greatest breadth of about 2J miles. This part of 

 the country has received but brief notice from Sir A. Eamsay in his 

 memoir on the " Geology of North "Wales " t? and from Dr. Hicks X^ 

 who claims a portion of the area for his Pre-Cambrian systems. 

 Several specimens from the district have been described by Professor 

 Bonney§ and the late Mr. Tawneylj, and the latter has also made 

 a few observations on the field-relations of some of the rocks ; but 

 with these exceptions we have no published information about the 

 western part of the Lleyn peninsula, though there are probably few 

 districts of equal size in Britain where so many interesting rock-types 

 are to be met with. 



The rocks will be discussed in the following order : — Granite and 

 Gneissic Granite ; Gabbro, Diorite, and Gneissic Diorite ; Diabase ; 

 Hornblende-Diabase ; Hornblende-Picrite ; and Dolerite. Some of 

 these, however, are but little developed, and, for most purposes, the 

 rocks of the district may be divided into two groups — an acidic, de- 

 veloped in the north and west, and a basic and intermediate in the 

 east and south. Mr. Tawney and Dr. Hicks have pointed out that 

 the map of the Geological Survey does not correctly indicate the 

 separation between the two groups, the extent of the " green- 

 stone " being unduly enlarged at the expense of the " syenite." The 

 accompanying sketch-map (fig. 1) is intended to show roughly the 

 limits of the two sets of rocks ; but much of the country is obscured 

 by drift, and accuracy is impossible without better topographical 

 maps to serve as a basis. 



* Sam Meyllteyrn on the Survey Map, one-inch scale, sheet 76. 



t Mem. Geol. Surv. Gr. Brit. vol. iii. 2nd edit. (1881). 



X Quart. Journ. Geol. Soc. vol. xxxv. p. 298 (1879). 



§ Geol. Mag. dec. ii. vol. vii. p. 207 (1880) ; Quart. Journ. Geol. Soc. vol. xxxv. 

 p. 306 (1879). 



II Geol. Mag. dec. 2, vol. vii. pp. 207-215, and p. 456 (1880) ; vol. x. pp. 65- 

 68 (1883) ; see also TeaU, 'British Petrography,' 1888. 



