1813.] On Veins. 



lar attention. All the different substances are deposited in 

 layers or plates parallel to the sides of the vein. And they follow 

 a determinate order. A particular layer constitutes the middle 

 of the vein ; and the very same substances in the very same 

 order are repeated on both sides of it to the v^^alls of the veioa 

 An example will make this structure evident. 



1 shall select for that purpose a vein described by Werner in 

 his treatise. It occurs in a gneiss rock near Freyberg, and 

 consists of nine layers, or four pairs, regularly arranged on each 

 side of the central one ; and each pair is composed of the same 

 substance, and has the same thickness. Next the walls of the 

 veins on both sides, there is a layer of quartz ; next to the quartz 

 is a layer of brown spar ; then follows a layer of galena, with a 

 layer of silver ore ] and the centre of the vein consists of a seam 

 of calcareous spar. Thus the position of the layers is as in the 

 following diagram : — 



=3 



8 



■13 



CO 



$-1 



C3 



Is 



la 



o 



o 



a 



When these complicated veins are traced downwards, it some- 

 times happens that the central seam disappears altogether. In 

 that case the two seams on each side of it coalesce into one, and 

 become henceforth the central seam of the vein. If we continue 

 our progress downwards, this new central seam may perhaps 

 disappear in its turn, and the two layers on each side of it 

 coalesce, and form a new central seam. This successive disap- 

 pearing of the seams continues as we proceed downwards, till at 

 last none remain ; except the layers which are attached to the 

 walls of the vein. These coinciding, form henceforth a simple 

 vein, which gradually becomes thinner and thinner, and at last 

 disappears altogether, and the vein terminates. 



It is true that only a very few well authenticated instances of 

 very complicated veins, that have been thus traced, can be pro- 

 duced ; but a good many veins, consisting of three layers, have 

 been wrought till the central seam disappeared, and the layers 

 next the walls united, and formed a single vein. The layers 

 next the walls of the vein are usually thinnest above, and become 

 thicker as we descend. 



6. It is no uncommon thing to find two or more veins in the 

 Vol. L N° V„ ^ Z 



