104 VEINS THEIR NATUEE AND ORIGIN. 



pockets, nests, strings, and plates, according to the manner 

 and abundance of its occurrence. The vein-stuff is usually 

 arranged in layers from the walls inwards, the centre being 

 generally occupied by a rib of ore, though not unfre- 

 quently hollow and lined with crystals. The whole matrix 

 has thus a striped or veined appearance, the stripes run- 

 ning up and down or parallel to the cheeks or containing 

 walls. It must be obvious from this description that a 

 vein is something very distinct from the rocks through 

 which it passes. If it pass through igneous rocks, its 

 stripes and colours contrast very strongly with the dark 

 uniform hues of these masses; if through sedimentary 

 rocks, its upward and downward course through their strata 

 at once arrests the attention ; and, generally speaking, its 

 sparry or crystalline texture is sufficient to define its thick- 

 ness and direction. Passing from below upwards, and 

 frequently ramifying, crossing, and intercrossing in many 

 directions, they look indeed like the veins in vegetable and 

 animal structures, and hence their appropriate and expres- 

 sive designation. 



The sparry matter which forms the bulk of the vein- 

 stone or matrix consists for the most part of quartz, car- 

 bonate and sulphate of lime, carbonate and sulphate of 

 baryta, or of alternations and admixtures of these the ore 

 occupying a subordinate part in ribs, strings, nests, and 

 pockets. These vein-stuffs seem to have been deposited first 

 on the cheeks, and then coating after coating towards the 

 centre, which is either solid like the rest of the matrix, or 

 hollow, as if there had been a deficiency of filling matter ; 

 and, in such cases, the cavity is lined with crystals shoot- 

 ing and pointing inwards. Having had room to assume 

 their independent forms, the crystals in these cavities are 

 often of great beauty, and it is usually from such vein- 

 spaces that the mineralogist obtains his rarest treasures. 



