CROSSING OF VEINS. 



289 



truth, I am inclined to believe, it would be found inferior in magnifi- 

 cence and splendour of mineral decoration, to the natural caverns in 

 the fluor mine. This mine is rarely visited by travellers : the de- 

 scent is safe, but, the roof being low in some parts, it is rather diffi- 

 cult of access. 



The pipe vein may be described as a tubular mass of ore and 

 veinstone, generally descending in the direction of the beds, and wi- 

 dening and contracting in its course. In reality, the pipe vein is a 

 variety of the flat vein, having the sides closed or twitched in, so as 

 to form a tube or cavity of irregular shape, and of very limited ex- 

 tent along the line of bearing, but descending to a great depth. 



One metallic vein often crosses or cuts through another, and dis- 

 places it : in such instances, it is evident that the vein which is cut 

 through, must be more ancient than that which intersects it. This 

 observation respecting the relative ages of veins was first made by 

 Mr. Pryce in his Mineralogia Cornubiensis. The different position 

 of veins is represented in Plate fig. 4., where a a is a vein which 

 divides in part of its course and unites again, and finally branches off 

 into small strings. In many instances these strings lead to a further 

 continuation of the vein ; perhaps this would be found to be the case 

 in all, were the workings carried on in the same direction, b b \s 

 another vein which cuts through the former, and has thrown the low- 

 er part of the vein a out of its course. It is obvious that the vein 

 a a was formed before the vein b b, which has upheaved the rock on 

 one side, with the lower part of the vein a. In Plate VII. fig. 4., a 

 small vein is represented, cut into three parts by the larger veins, a 

 and b. Sometimes one vein crosses another without changing the 

 direction ; and if they both have nearly the same inclination, viz., 

 dip nearly to the same point of the compass, they are generally rich- 

 er near their junction, as at b, Plate VIl. fig. 4. When a number of 

 veins cross each other at one place, they sometimes form a cone or 

 mass of ore of vast size, widening as it descends. Such are called 

 accumulated veins. They occur in the metalliferous limestone of 

 Durham and Northumberland. When one vein crosses another in 

 an opposite direction, they often are found poorer in ore near the 

 junction. Fig. 3. shows a ground plan of the veins b b c c, cut 

 through nearly at right angles by another vein or cross course : in 

 such instances the veins bb c c become poorer ; but this is not uni- 

 versally the case. 



The direction of rake veins is not very regular. In England, the 

 principal veins generally run nearly east and west, and north-east 

 and south-west ; but have, frequently, undulations and deviations 

 from a straight line : the most powerful veins are more regular in 

 their course than smaller ones. Where two veins in the same dis- 

 trict have the same direction, or run parallel, it is observed that their 

 contents are similar ; but where they run in different directions, the 

 contents vary. Molina, in his interesting History of Chili, mentions 



37 



