162 Messrs. Vou Oeynhausen and Voii Declien on the 



Gwennap, the country between St. Day, Camborne and Hel- 

 ston; the parishof St. Just; the neighbourhood of St.Austle, — 

 will give as many instances of this fact as there are mines 

 opened in them. Even the adjacent part of Devonshire affords 

 many instances of this rule in the rich mine of Wheal Friend- 

 ship near Tavistock. The description of the different kinds 

 of veins that occur in Cornwall, given by Mr. Carne (Trans, 

 of the Geol. Society of Cornwall, vol. ii. p. 49), is so complete 

 and so exact, that we have not any thing to add. We only 

 beg leave to offer in the following lines our observations on 

 some cases of granite veins which are found at the junction of 

 this rock and the killas, and which we must consider as very 

 worthy of the notice of the geologist; since they seem to throw 

 some light on the relation of the granite to the overlying rocks. 



The greatest part of the granite contains only a small por- 

 tion of mica ; and that may be also the reason that scarcely any 

 rock occurs here which may deserve the name of gneiss : it 

 consists of white or grayish quartz and felspar in nearly equal 

 portions, and of large white twin-crystals of felspar like those 

 of Carlsbad, which give the upper rock a porphyritic appear- 

 ance. Schorl and also pinite occur very frequently in this de- 

 scription of granite. The colour of the felspar is in some in- 

 stances reddish. This granite does not incline to decomposi- 

 tion ; and China stone or China clay are very rarely found in it. 

 The most striking facts of the granite veins we observed in 

 the Land's End district, at the junction of the most western 

 granitic mass. 



I. Mousehole. — Near Mousehole, on the south of Penzance, the 

 granite appears below the killas, which must be considered rather 

 as greenstone than as slate. At low-water is to be seen the junc- 

 tion of both rocks dipping to the north-east ; it is not an uni- 

 form plane, but protuberancesof thegranitepierceintothegreen- 

 stone, Jhr in i?ig here and there a nearly i^erpendicidar jxmction,al- 

 though the deviationfrom a plane is not of consequence for the ge- 

 neral inclination of the junction betvoeen both rocks. A (ground) 

 plan of this spot is represented by fig. 1. (Plate II.) Several veins 

 of granite coming out from the main body of the granite here 

 traverse the killas. The vein (A) is one of the largest, from 

 3i to 10 feet wide; it runs east and west, underlying rapidly 

 to the north. The granite in it is fine-grained, has a I'eddish 

 hue, a very close texture, contains only a very small portion of 

 mica ; therefore it differs in some degree from the granite of 

 the main body, which contains large crystals of felspar : but 

 it may be remarked that near the junction of both rocks these 

 large crystals are sometimes wanting, and that the texture 

 of the granite is more compact here than further from the 



killas. 



