Dr. Mac Culloch on Mineral Veins. 191 



aspect. In some cases, this substance is clay; in others, quartz 

 is found ; and, not unfrequently, it consists of a conglomerate 

 formed out of fragments of the bounding rocks, united by va- 

 rious crystalline and earthy substances. It is common, in these 

 cases, to find that the including rock is more or less decom- 

 posed and altered, at its junction with the vein. It has also 

 been observed, that large detached fragments of the neighbour- 

 ing rock are sometimes included within the body of the vein. 

 In some cases, this occurrence presents an interesting varia- 

 tion ; as, when a vein traversing schist and granite together, is 

 found to contain fragments of the former within the space 

 bounded by the latter, and the reverse. This fact serves to 

 prove the extent of the revolutions, of a mechanical nature, 

 which must have taken place in the vein ; either at the time, or 

 after the period, of its formation. 



It is unnecessary to enumerate all the earthy minerals which 

 have been found in veins ; but the most common are quartz 

 calcareous spar, barytes, and fluor. These, like the metallic 

 substances, are found in different parts of the vein, and are 

 crystallized in different forms, wherever cavities are present. 

 The metallic minerals are found variously disposed ; sometimes 

 lining similar cavities in their crystalline forms ; at others, col- 

 lected into lumps, or deposits, in different parts of the vein ; 

 and at others, again, more generally diffused among the gene- 

 ral mass of materials. In some instances, only one metal is 

 found in a vein, in others, two or more ; and these are some- 

 times distinctly separated, at others intimately mixed, so as to 

 be a source of much trouble to the miner. It is occasionally 

 found that the minerals, whether metallic or earthy, are ar- 

 ranged in layers parallel to the sides of the vein ; and, in some 

 of these instances, there is, further, a perfect correspondence 

 on the opposite sides. Such, also, is the capricious disposition 

 of the metals, that they sometimes disappear altogether, after 

 having abounded through a large space ; so that it becomes 

 necessary to abandon a mine that had once proved very pro- 

 fitable. It is owing to these perpetual variations in the con- 

 tents of mineral veins, that the characters of particular mines 

 Vol. XV. O 



