Ca xxxvni] origin" of metalliferous veests. 621 



It is clear that, after the gradual introduction of the tin and other 

 substances, the second rent (b V) was produced by another fracture ac- 

 companied by a displacement of the rocks along the plane of b b. This 

 new opening was then filled with minerals, some of them resembling 

 those in a a, as fluor-spar (or fluate of lime) and quartz ; others diffe- 

 rent, the copper being plentiful and the tin wanting or very scarce. 



We must nest suppose the shock of a third earthquake to occur, 

 breaking asunder all the rocks along the line c c, fig. 711 ; the fissure .in 

 this instance, being only 6 inches wide, and simply filled with clay, de- 

 rived, probably, from the friction of the walls of the rent, or partly, 

 perhaps, washed in from above. This new movement has heaved the 

 rock in such a manner as to interrupt the continuity of the copper vein 

 (b b), and, at the same time, to shift or heave laterally in the same di- 

 rection a portion of the tin vein which had not previously been broken. 

 Again, in fig. 712 we see evidence of a fourth fissure (d d), also filled 

 with clay, which has cut through the tin vein (a a), and has lifted it 

 slightly upwards towards the south. The various changes here repre- 

 sented are not ideal, but are exhibited in a section obtained in working 

 an old Cornish mine, long since abandoned, in the parish of Kedruth, 

 called Huel Peever, and described both by Mr. Williams and Mr. 

 Carne.* The principal movement here referred to, or that of c c, fig. 

 712, extends through a space of no less than 84 feet ; but in this, as in 

 the case of the other three, it will be seen that the outline of the country 

 above, cZ, c, b, a, &c, or the geographical features of Cornwall, are not 

 affected by any of the dislocations, a powerful denuding force having 

 clearly been exerted subsequently to all the faults. (See above, p. 69.) 

 It is commonly said in Cornwall, that there are eight distinct systems of 

 veins which can in like manner be referred to as many successive move- 

 ments or fractures ; and the German miners of the Hartz Mountains speak 

 also of eight systems of veins, referable to as many periods. 



Besides the proofe of mechanical action already explained, the opposite 

 walls of veins are often beautifully polished, as if glazed, and are not 

 unfrequently striated or scored with parallel furrows and ridges, such as 

 would be produced by the continued rubbing together of surfaces of un- 

 equal hardness. These smoothed surfaces resemble the rocky floor over 

 which a glacier has passed (see fig. p. 127). They are common even in 

 cases wnere there has been no shift, and occur equally in non-metalliferous 

 fissures. They are called by miners " slicken-sides," from the German 

 scklichten, to plane, and seite, side. It is supposed that the lines of the 

 stria? indicate the direction in which the rocks were moved. During one 

 of the minor earthquakes in Chili, which happened about the year 1840 

 and was described to me by an eye-witness, the brick walls of a building 

 were rent vertically in several places, and made to vibrate for several 

 minutes during each shock, after which they remained uninjured, and 

 without any opening, although the line of each crack was still visible. 



* Geol. Tram. vol. iv. p. 139 ; Trans. Roy. Geol. Society, Cornwall, vol. ii. p. 90. 



