1 72 ON THE MINERAL AND METALLIFEROUS [Ch. IX. 



These veins are exceedingly numerous, and parallel with 

 each other; and are almost perpendicular. They are in 

 general small, very seldom exceeding a foot in thickness. The 

 quartz or matrix of these veins is frequently crystallized in the 

 central parts, so as to form drusy cavities or elongated open- 

 ings, which are often lined with crystallised mica, tin ore, 

 tungstate of iron, and other minerals. In their course they 

 send off numerous branches, which often ramify in the rock, 

 and are sometimes reunited with the main vein. They do 

 not run in a straight line, but in one which is more or less 

 undulating; and are not of an equal breadth throughout, 

 expanding, in some places, into large bunches, and in others 

 contracting to the size of a string ; and this irregularity is 

 more prevalent in the slate than in the granite. The rock, 

 whether slate or granite, adjacent to these veins, is always 

 more quartzose than at a distance therefrom, so that they 

 gradually pass into each other ; and the boundary of the veins 

 is more seldom marked by the joints or open seams, or walls 

 as they are usually termed, than commonly happens in other 

 localities. Heaves are of rare occurrence : but it may be 

 remarked, that, in one instance, a vein seems to have been 

 heaved about three inches by the N. and S. joint of structure, 

 but the same line intersects other parallel veins without pro- 

 ducing the same effect. 



Near the pier at Mousehole, also in Mount's Bay, a large 

 quartz-vein traverses both the granite and the slate, and may 

 be examined for more than 150 feet in length. (See Jig. 12.) 



It will be perceived, that the width of this vein is much 

 exaggerated, in order to render its nature more visible ; for, if 

 it had been drawn in proportion to the above scale of its 

 length, it would have been reduced to a mere line. Its 

 average size is about one foot in thickness : at the southern 

 end it sometimes attains to fifteen inches, and then generally 

 contains large drusy cavities lined with quartz crystals ; but 

 near such enlargements it will suddenly contract its dimen- 

 sions to three, or even two inches ; and in the middle of its 



