492 TROF. T. G. BOKN^ET AND MAJOR-GEN. C. A. M*^MAnON 



calm weather. We had to restrict ourselves to what could be seen 

 by working along their face ; so we cannot attempt more than to 

 give a general sketch. Gabhro of the ordinary type can be traced 

 at intervals on the slope above the cliffs, certainly to within a furlong, 

 and probably to within a hundred yards of the water's edge. 



These cliffs, as far as we could get at low water, were found to 

 consist of a greenstone (epidiorite) *. This is cut by veins of a fel- 

 spathic rock, which must be classed with the actinolitic gabbros, 

 since it consists mainly of moderately coarse plagioclastic felspar and 

 small patches of an actinolitic mineral, probably replacing augite. 

 Sometimes the veins are very thin and might be infiltrations. One 

 or two indeed appeared to contain quartz, and if so the rock might 

 almost be called a felspathic pegmatite. This rock and the green- 

 stone are closely welded ; the former differing markedly from the 

 normal gabbro, which, as mentioned above, occurs on the hillside at 

 a short distance. Both are traversed by dykes of compact green- 

 stone, maeroscopically indistinguishable from that which cuts the 

 ordinary gabbro about Manacle Point, but in one of them a por- 

 phyritic structure occurs. This variety, under the microscope, is 

 found to be a compact epidiorite, containing large crystals of plagio- 

 elase felspar. There can be little doubt that the matrix was formerly 

 a minutely crystalline, or possibly even a vitreous, basalt. 



The relations of the first and second rock much resemble those 

 of the Granulitic Group, but in the present state of our knowledge it 

 would be rash to do more than note the resemblance ; certainly, as 

 the gabbro-like rock proves to be so different from the ordinary 

 gabbro of the district, we are not justified in regarding them, with- 

 out further proof, as of the same age. One difficulty in the identi- 

 fication of the first two rocks with the Granulitic Group is that they 

 are little, if at all, foliated, while the crags facing them across the 

 narrow cove are " slabby," and sometimes Avell-banded, hornblende- 

 schist. Much minute study and repeated visits will be necessary in 

 order to clear up the difficulties of this section of the coast. 



YI. Other Intrusive Rocks. 



As has been often remarked, the schists, the serpentine, and the 

 gabbro are alike cut by intrusive dykes. These are more numerous 

 along the east coast than on the west, being especially common 

 between Caerleon Cove and Porthoustock. To classify and describe 

 them fully would require a separate memoir. On the west coast 

 the serpentine is not seldom cut, as described in Prof. Bonney's 

 paper, by dykes and small masses of a reddish, rather fine-grained 

 granite, which generally is distinguishable, maeroscopically and mi- 

 croscopically, from the granitic rock of the Granulitic Group, which, 



* It consists chiefly of plagioclase felspar in good condition (probably labra- 

 dorite) and a fibrous hornblendic mineral, clearly of secondary origin. Probably 

 the original was a rather fine-grained dolerite with a structure inclining to 

 ophitic. Larger idiomorphic crystals of rotten felspar occur rather sparsely. 



