ON THE ROCKS SURROUNDING THE LAND'S-END MASS OE GRANITE. 407 



43. On the Metamorphic Rocks surrounding the Land's-End Mass of 



Granite. By S. Allport, Esq., F.G.S. (Read June 21, 1876.) 



[Plate XXIII.] 



In the following notes I propose to lay before the Society the results 

 of a microscopical examination of certain metamorphic rocks sur- 

 rounding the Land's-End granite. In doing so my principal object 

 will be to point out the structural and mineralogical changes pro- 

 duced in clay-slates and certain igneous rocks by the intrusion of a 

 mass of granite, and also to compare the phenomena of contact- 

 metamorphism with those produced by other agencies in rocks of 

 similar character at a distance from the granite. 



On referring to the map of the Geological Survey of Cornwall, it 

 will be seen that in the vicinity of Penzance the granite is sur- 

 rounded by a belt of altered Devonian slates extending from St. Ive's 

 Bay on the north coast to a point a little beyond Mount's Bay on 

 the south ; they also occur along the coast north-east of Cape Corn- 

 wall, and reappear in the same direction at Porthmear Cove and 

 near Zennor. The so-called greenstones occur in four distinct 

 groups, also round the outskirts of the same mass of granite, viz. 

 around Penzance and in the other localities just mentioned. In all 

 cases they are represented as occurring in the altered slates either at 

 a short distance from the granite or in actual contact with it. 



A microscopical examination of both these groups shows at once 

 that they have all been highly metamorphosed, and that the 

 alteration varies greatly in character and extent according to the 

 nature of the rock and its distance from the granite. The change 

 observable in the so-called greenstones, for example, differs essen- 

 tially from that produced by the intrusion of the granite among the 

 clay-slates. In specimens of the former, taken at some distance 

 from the granite, the alteration consists principally in the formation 

 of pseudomorphs quite similar to some described by me elsewhere in 

 the Carboniferous dolerites of Scotland ; while, nearer the granite, 

 similar rocks have been completely decomposed, and hornblende has 

 largely replaced the original constituents. 



The action of the granite on the slates in immediate contact with 

 it, however, has been far more direct and energetic, the result being 

 the frequent introduction of new minerals, and the conversion of an 

 ordinary clay-slate into a crystalline foliated rock. 



On approaching either of the granite masses of Cornwall, one 

 cannot fail to recognize the fact that the clay-slates gradually 

 assume a different character : they become more and more indurated 

 and are traversed in all directions by numerous quartz-veins ; they 

 frequently become more or less micaceous, schorl begins to make its 

 appearance, and at the junction of the two rocks their slaty cha- 

 racter has in many cases been completely obliterated. In the 

 neighbourhood of Penzance the petrological relations of the granite 

 to the adjacent sedimentary deposits may be readily observed in 



