402 NATURE OF RIBS OF ORE. 



colour and composition. Werner adopted the notion of the unequal 

 antiquity of the vertical layers of the vein so implicitly as to speak 

 of the middle ribs as always younger. It is difficult to resist 

 this impression, especially \vhen, in addition to the circumstance of 

 the succession of the laminae, we observe that these laminae are so 

 crystallised as to turn their free terminations towards the centre of 

 the vein, and in that direction to imprint the next layer with their 

 own forms, just as crystals forming in a vessel shoot their point toward 

 the part still remaining liquid, and in that direction are covered 

 by the subsequently formed crystals. Very similar inferences are 

 suggested by certain agates, and more distinctly by geodes in basalt, 

 and the crystallised cavities in limestones, in the interior of shells, 

 &c. ; in which cases the hollow towards which the crystals pointed 

 still remains unfilled. 



This first impression becomes somewhat modified when, instead of 

 confining ourselves to a cabinet specimen, we examine the whole 

 extent of a mine ; for here, in the first place, it is very often found 

 that the regular succession of minerals from the side to the centre is 

 a limited though repeated phenomenon ; that the rib of ore is of 

 short horizontal, and sometimes still shorter vertical extent, diminish- 

 ing to nothing, or diffused in small grains through the contiguous 

 spars ; that different metals are found in the same vein at different 

 depths, and at distant points along its course ; and that both the 

 quantity of metal and the presence of spars are dependent on the 

 hardness, and perhaps on some other physical properties of the rocks 

 which the vein divides and replaces. 



Chemical Relations of Metals. The phenomena of crystallisation 

 before alluded to can hardly be thought to prove the successive intro- 

 duction of the mineral laminas into the vein ; though very probably 

 they do demonstrate the order of crystallisation of these substances. 



In some cases we observe indications that one kind of mineral has 

 been formed round another as a nucleus ; as, for example, sulphide 

 of copper round iron pyrites in a part of Caldbeck Fells, Cumberland, 

 and more frequently in many places carbonate of copper and carbonate 

 of lead round the sulphides of those metals. As we have remarked, it 

 is very often the case that the metallic matter of the vein is collected 

 into the middle and forms there a distinct tabular mass, called a rib of 

 ore, more rarely it is disseminated into the gangue. Generally, only one 

 kind of metal abounds in the same part of the vein, but the same vein 

 may yield lead above and copper below, copper above and tin below, 

 or lead in one place and copper in another. The observation is fre- 

 quent that ore is collected into certain vertical portions of a vein 

 which are worked above and below a level, and between which little 

 but vein-stuffs is found in the horizontal drift. There is a vague 

 notion amongst miners that veins are most productive in the deep 

 workings, and it is at least probable that they are less rich very near 

 the surface. 



Association of Minerals. Werner insisted on the fact, that cer- 

 tain associations of minerals can be traced in veins. He noticed the 



