54- DESCRIPTION OF THE PRIMARY ^Ch. ^. 



The granitic el vans (species of eurite) are very abundant 

 between St. Agnes and Cligga Point, on the north coast, 

 where they may be seen running in regular beds, alternating 

 with the schistose rocks into which they gradually pass : some 

 parts of these el vans are very shorlaceous and quartzose ; and 

 being on this account more durable than the felspathic kinds 

 and the slate, they project on the shores in very abrupt 

 ridges. 



On the shore at Swan-pool, near Falmouth, there is a 

 large elvan-course, the upper part of which in the cliff is ex- 

 tensively decomposed into a state of clay : its course on the 

 shore may be traced for several fathoms in length ; but what 

 is more particularly worthy of remark is, that near low-water 

 mark it abruptly terminates, but immediately adjoining it is 

 seen running in a parallel direction at a distance of about 

 twenty feet from its original course, which is exactly the 

 width of this elvan. This is an excellent illustration of what 

 the miners call a heave, and is interesting, as this pheno- 

 menon is not of such frequent occurrence as in the case of 

 veins : in this instance the elvan has the appearance of having 

 been intersected by a thin course of clay, or jflukan, which, 

 however, gradually tapers or dies away on the land side. At 

 Mousehole, in Mount's Bay, a very large vein, or rather a 

 course * of felspar porphyry, exhibits heaves by veins of quartz : 

 the course is visible, at low water, for more than a hundred 

 and fifty feet in length, and is sufficiently important to de- 

 mand a particular description, which may be rendered more 

 intelligible by the sketch, (fig. 2.) 



The bearing of this course is a little S. of W. and N. of E., 

 on an irregularly undulating line, dipping towards the N. at 



* A large vein of a similar nature, at Cape Cornwall, ought to be considered 

 as an elvan-course, according to Mr. Carne ; but it matters not in which 

 light these elongated granitic masses are viewed, because no line of distinction 

 can be drawn between them. It is worthy of remark, however, that elvan. 

 courses generally run parallel with the strike of the laminae of the slates j whereas 

 at both these places they cross them diagonally. 



