168 ON THE MINERAL AND METALLIFEROUS [Ch. IX. 



minute veins are more or less simple in their composition : 

 very frequently they consist, for the most part, of a single 

 mineral, the nature of which generally bears some relation to 

 the constitution of the containing rocks : thus, quartz-veins 

 are found in all rocks, which might be expected, since all 

 rocks abound more or less in silica; but they are most 

 frequent in those rocks wherein this earth predominates ; so, 

 likewise, calcareous veins occur in rocks, into the composition 

 of which lime enters ; and veins of asbestos, steatite, and 

 other magnesian minerals, characterise serpentine, euphotide, 

 and other rocks of a congenerous nature ; and, lastly, what- 

 ever may be the peculiar and distinguishing mineral of any 

 series of granitic or schistose rocks, whether hornblende, 

 chlorite, shorl, actynolite, or mica, small veins and patches 

 of the same substances impart to the mass an appearance 

 more or less variegated. Sometimes, however, the substance 

 of these veins is compound, exhibiting either distinct or 

 homogeneous crystalline mixtures of two or more minerals, 

 which very commonly are only varieties of the rock in which 

 they occur : thus, veins and irregular portions of fine-grained 

 granite, of syenite, of shorl-rock, and of other granitic 

 species, are frequently completely enveloped in various kinds 

 of granite; and hornblende-rock, actyiiolite-rock, and ether 

 members of the schistose group, contain small veins of a more 

 crystalline nature imbedded in the homogeneous varieties, 

 and vice versa ; these occurrences, however, are not very con- 

 spicuous, unless rendered obvious by a partial decomposition, 

 by which the harder or more crystalline parts are brought 

 into alto relievo on the surface of the blocks. 



Such is the composition of the small or concretionary 

 veins of the primary rocks : the next points for consider- 

 ation are, their connection with the rock, and their structure- 



A fresh fracture, or, indeed, the external surface, if the 

 rock be of a durable nature, shows that the substance of 

 these veins is often intimately blended with that of the rock ; 

 so that it is impossible to say where the one begins and the 



