132 POSITION AND NATURE OF [Ch. VII. 



fined to distinct concretions ; and this more generally obtains 

 when the granite abounds in crystals of felspar at the 

 junction: but even this does not constantly hold good; for 

 sometimes these prophyritic crystals, as at Polmear Cove, are 

 also contained in the dark-coloured slate, for many inches 

 from the point of contact of the two rocks, and this intimate 

 union is farther displayed by this phenomenon occurring in 

 the same individual block or concretion. Oftentimes the 

 granite is very quartzose at its junction, and gradually passes 

 into quartzose varieties of slate ; or, as at Forth Just, Cape 

 Cornwall, this transition takes place through the medium of 

 a large quartz vein (Little Bound's Lode), into which the 

 granite and the slate, on either hand, distinctly graduate. In 

 short, the granite and slate of Cornwall, at their junction, 

 are frequently so similar, both in composition and concre- 

 tionary structure, that the detached blocks, as they lie side by 

 side, cannot at a little distance be distinguished from each 

 other ; the darker colour of the slate, and its tendency, often 

 slight, to break into laminae, are sometimes the only differences 

 to be detected on a closer inspection. 



We will now examine whether the junctions of other coun- 

 tries afford similar facts. M. de Caumont, in his account of 

 the department of Manche, in France, has detailed the ap- 

 pearances of the passage of slate into granite. Near Cher- 

 bourg, a greenish schist contains fine particles of felspar, 

 which gradually become more and more abundant; at the 

 same time grains of quartz become visible, and the rock 

 assumes a granular texture : and, in these places, it resembles 

 a kind of gneiss or talc-schist, or, rather, the nodular steaschist 

 of Brongniart. At last, the grains of felspar and quartz, 

 elongated and imbedded between the laminaB of the schist, 

 gradually lose this regular arrangement, acquire a more 

 crystalline form, till the mass becomes a granite or syenite.* 



The junction of the granite and mica-slate is thus described 



* Bulletin de la Soc. Gdol. de France, tomeiii. p. 11. 



