Ch. IX.] VEINS IN PRIMARY ROCKS. 179 



parallel, and sometimes coincident, to the sides of the larger ; 

 but far more frequently to the subsequent formation of seams 

 or fissures, by the alteration of the rock at the junction of the 

 veins, resulting from decomposition, the effects of the percola- 

 tion of water, or of the action of the elements. How can we 

 otherwise account for the fact, that many parts of those veins, 

 exhibiting what has been called regular walls, are intimately 

 connected with the adjacent rock, not only as it were by a 

 mechanical union, but often by a transition of mineral com- 

 position, so that in granite the union is generally effected by 

 the rock gradually becoming more and more quartzose, and 

 in the slate it is also accomplished by the latter undergoing a 

 like change ? Sometimes indeed the vein itself, at these 

 points of union, appears to partake of the nature of the con- 

 taining rock ; but much more commonly it entirely includes 

 portions of the rock of various dimensions, according to the 

 size of the vein. When the portion of rock thus circum- 

 stanced is considerable, it appears, as seen on the sea- shore, 

 as if the vein had divided into two branches, which embrace a 

 large angular piece of rock and then reunite ; but, in mines, 

 these masses of detached rocks, like the minute portions, are 

 found to be completely enveloped ; and this is of such common 

 occurrence, that the vein is said by the miners to have taken 

 horse. Now, these included portions of rock, whether they be 

 small or whether they be large, exhibit the same relations to 

 the vein as the latter does to the main mass of rock ; that is, 

 sometimes they appear to be clearly defined and distinctly 

 separated from the quartzose part of the vein, but, at other 

 times, they pass into the substance of the vein. 



There is scarcely a mine in Cornwall in which the veins do 

 not contain more or less of these angular portions of the 

 adjacent rock, and in which this phenomenon of transition 

 may not be observed : and sometimes these portions of rock 

 so predominate over the quartzose part of the vein, that the 

 latter seems only to cement the former together : and, indeed, 

 some lodes, as at Lanes cot mine, have been found to consist 



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