210 GENERAL REMARKS [Ch. X. 



the veins. So that granite-veins, and el van-courses, only 

 differ from each other in size ; just as small mineral veins 

 differ from those which are of considerable extent, and metalli- 

 ferous : the latter and elvan-courses commonly continue for 

 so great a distance, that they have been supposed not to 

 terminate, to have an indefinite length ; but this is not matter 

 of fact, for it is well known that both kinds frequently do 

 terminate, at one or both ends, in minute strings which 

 gradually disappear, even when the courses and veins are of 

 no inconsiderable extent ; thus, the quartz metalliferous veins, 

 at Mousehole, St. Just, and, indeed, on all the shore of the 

 Land's End district, are thus circumstanced ; and the elvan- 

 courses of Cape Cornwall and Mousehole terminate imper- 

 ceptibly, the latter at one, and the former at both extrem- 

 ities. 



Both the granitic and mineral veins of Cornwall gradually 

 pass into the rocks which they traverse ; and both present the 

 same appearances of heaves, slides, and similar phenomena 

 commonly referred to motion, when veins of the same or of a 

 different class interfere with each other. 



Having now concluded our general sketch of the primary 

 formations, in which it has been attempted to show that the 

 rocks of Cornwall are analogous to those of other granitic 

 countries, in composition, structure, arrangement, and modes 

 of association, we now proceed to the only remaining con- 

 sideration concerning them ; viz. the nature of their origin. 

 Hitherto our subject has been descriptive ; but, at the next 

 step, it will become speculative ; since a notion of the origin 

 of these rocks can only be arrived at by deductions from 

 facts : the conclusions thus formed, must, of course, be subject 

 to frequent revisions, as we progressively obtain a more en- 

 larged knowledge of Nature by the accumulated experience 

 of successive observers. In the following chapters it is pro- 

 posed to put the prevailing theory concerning the origin of 

 the primary rocks, to the test of the facts which have been 

 collected together in the preceding details : we shall, there- 



