xii 
line of junction between the different rocks. Joints maintaining 
different directions intersect the several varieties of slate; but 
whether they meet at similar angles to the joints which traverse 
the granite,* is, perhaps, matter for further inquiry. 
Elvan is, for the most part, disposed in veins or (courses) dykes, 
which sometimes measure a few feet only, but are commonly 
several fathoms, in width; occasionally also—though much less 
frequently—it occurs in isolated (bunches) masses.t Elvan-courses 
traverse granite as well as slate without interruption ; and, in one, 
well-known, instance, two Jodes, at least, are intersected by an 
elvan.t Elvan-courses frequently conform, both in direction and 
in dip to one series of joints in the rocks they penetrate ;§ but 
they rarely interlie the cleavage-planes of the slate.§ Off-shoots 
from them now and then—but less frequently than branches from. 
the Jodes—extend into the adjoining strata.|| Whilst in the slate 
they generally consist of compact felspar and quartz, mixed at 
times with schorl and—perhaps less frequently—with mica ; en- 
close buff, pink or dove-coloured crystals of felspar and double- 
pointed crystals of quartz which sometimes seem to have suffered 
attrition. During their course through the granite, felspar and 
quartz still prevail, mica and schorl abound, and imbedded crystals 
are still numerous, but the basis is mostly of finer grain than in 
the slate. In both rocks, however, the elvan is usually coarser 
and more porphyritic towards the middle than at the sides. Near 
the surface—especially in the slate series—concentric envelopes of 
ferruginous matter occasionally enwrap masses of rock which 
differ in no respect from the ordinary elvan which surrounds 
them.{ Joints without number traverse the elvans in all direc- 
tions, dividing them into small blocks of irregular shapes.** In 
some varieties these joints are faced with schorl; in other, with 
* Boase, Cornwall Geol: Trans: iv., pp. 428-429. De la Beche, Report, 
pp. 272-274. Henwood, Cornwall Geol: Trans: v., pp. 168-174, Table 99; 
viil., pp. 673-674. 
+ Borlase, Natural History, p. 92. Pryce, Mineral: Cornub: p. 95. 
Thomas, (R.), Report, pp. 17, 49. Carne, Cornwall Geol: Trans: ii., p. 80. 
Henwood, Edin: New Phil: Journal, xxii., p. 166; Cornwall Geol: Trans: 
v., p. 166; vill., p. 660; Pick and Gad, p. 52-54. Ante, p. 9. 
+t Hawkins, Cornwall Geol: Trans: i., p. 151; Pl. v. Henwood, Ibid, 
v., p. 128; Table lxxxv. 
§ Henwood, Ibidv, p. 165. Holl, Quarterly Journal of the Geological 
Society, xxiv., p. 415. 
|| Henwood, Cornwall Geol: Trans: v., p. 166. 
q Dela Beche, Report, pp. 173-192. Henwood, Cornwall Geol: Trans: 
v., pp. 160-168. 
** Borlase, Natural History, p 92. Thomas, (R.), Report, p.17. Boase, 
Cornwall Geol: Trans: iv., p. 401.. Henwood, Ibid v., p. 164; viii., p. 652. 
