340 J. ARTHUR PHILLIPS ON IHE ROCKS OF 



of one or more efforts of vulcanicity which took place after the con- 

 solidation of the more superficial portions of the granite, but by 

 which the still liquid material from below was forced upwards. 



The lode- fissures containing ores of tin and copper were evidently 

 formed after the consolidation of the elvans which they intersect ; 

 and, finally, cross veins must necessarily be of later date than any 

 lodes through which they pass. It must, however, be borne in mind 

 that the fact of one vein having been apparently displaced by another 

 is by no means a proof that its including fissure is older than that 

 of the vein which appears to have displaced it. The two fissures 

 may have been contemporaneous, and their having been open at the 

 same time may have resulted in a local enrichment of the kind so 

 frequently noticed in such localities. It is even possible that, in some 

 cases, the so-called cross vein may be of anterior date to that which 

 it appears to have displaced. 



The veinstones of Cornish lodes are chiefly siliceous, consisting of 

 quartz, capel, chalcedony, &c, together with such silicates as schorl 

 and chlorite, one or other of which forms an essential ingredient of 

 all capels. Fluor-spar, calcite, sulphate of barium, &c. also fre- 

 quently occur, as well as iron pyrites and various other minerals and 

 ores. In some cases these materials are arranged, in duplicate, in 

 bands parallel with the walls of the vein, and following a regular 

 sequence. At other times the vein-matter is composed of a breccia 

 produced by the cementing together, by some siliceous, calcareous, 

 or metalliferous material, of angular masses of the enclosing rock. 

 These included fragments of " country " are frequently enveloped 

 by successive accretions of crystalline quartz, each distinguished by 

 some peculiarity of either structure or colour. 



It is also important to observe that when the included fragments 

 consist of slaty matter, their planes of cleavage usually correspond 

 with those of the enclosing rock, and that, when " horses " or large 

 masses of country are enclosed in a vein, there is generally an exact 

 accordance, with respect to bedding, cleavage, and other character- 

 istics, between it and the immediately adjoining wall-rock. 



Druses, or " vughs," of more or less considerable dimensions are 

 met with in some part of almost every lode ; and the interiors of 

 these are often lined with elongated crystals of quartz, on the sur- 

 faces of which the most delicate isolated crystals of calcite, fluor- 

 spar, or of some metalliferous mineral have subsequently been 

 deposited. Chalcedony sometimes alternates with quartz in the 

 banded or " comby " portions of veins ; and capped crystals of 

 the latter mineral are found, of which the several parts, being- 

 separated by a thin layer of clay, are readily detached. Stalactitic 

 quartz not unfrequently occurs in Cornish lodes. 



These and numerous other facts indicate that the contents of 

 mineral veins have been deposited by aqueous agencies ; but at what 

 temperature these, in each case, operated, there is not evidence to 

 show, since the ratio of the vacuity to the liquid in the fluid-cavities 

 of the same veinstone appears, like those in rocks, to be subject to 

 considerable variation. That such deposits have sometimes been 



