845 



MINERAL VEINS. 



MINERAL VEINS. 



846 



exist in these situations, previous to the formation of the veins, is 

 often evident from the fact that the sparry or metallic matters lie in 

 the interior of closed originally hollow shells, or fill lines of fracture 

 and fissure across corals, shells, and fishes. It is further evident from 

 the fact that along the line of a rake-vein the strata are commonly 

 found to have been violently displaced, and moved upwards or down- 

 wards many inches, feet, yards, or fathoms, even to fifty or more 

 fathoms, and reunited in this broken state by the subsequently intro- 

 duced mineral crystallisations. This consideration supplies ua with a 

 definition of veins, identical, except by the omission of the words in 

 brackets, with that given by Werner : " Veins are rents produced in 

 rocks, which were afterwards filled [from above] with certain sub- 

 stances." (' On Veins,' p. 57.) 



In such cavities, spars of several kinds, as carbonate of lime, quartz, 

 sulphate of barytes, sulphate of strontian, are frequently found, 

 entirely unconnected with metalliferous districts, and as a usual and 

 ordinary occurrence. In metalliferous districts the same and other 

 minerals occur, associated with carbonates, phosphates, arseniates, 

 sulphurets, &c. of lead, iron, copper, silver, &c., in almost every pro- 

 portion. Between the most valued mineral vein, such perhaps as 

 ores of precious metal, and the unprofitable massses of ' dead spar,' 

 which occur in almost every limestone-quarry, there is an almost 

 uninterrupted gradation ; they must all be taken into the induction of 

 phenomena, as a basis of theory ; yet there are peculiarities in the 

 geograph n,il and geological relations of the metalliferous or true 

 mineral veins which require separate classification, and justify some 

 special inferences touching the local conditions and geological times of 

 their formation. 



1. In a mining district, however rich, not all the cavities in the 

 rocks yield metallic minerals, even though they contain spars such as 

 often accompany these. Generally the ores of lead, copper, silver, &c., 

 are limited in their occurrence to such great fissures of the rocks as 

 are accompanied by displacements of the masses of rock which bound 

 the fissure. Thus great fissures, Nos. 1 and 2, in the figure, which 



are marked by dialocations, may be richly filled with valuable ores, 

 and yet the detached cracks and fissures (j) may contain only unpro- 

 fitable spare or thin partings of clay. 



2. When, a<i frequently happens in mining districts, veius accom- 

 panied l>y dislocation cross each other, the veins commonly become 

 either richer or poorer in metal, and exhibit other peculiarities, about 

 the junctions. 



3. Such veins which cross may be of quite different natures : No. 1, 

 for example, may yield copper-ore, and No. 2, lead-ore. This differ- 

 ence of contents in adjacent or meeting veins is even very frequently 

 the case when the direction of the veins on the surface differs by a 

 quadrant of a circle. Thus in Cornwall, veins which pass east and 

 west may yield tin or copper, and those which run north and south 

 produce lead. In Aldstone Moor, veins running east-north-east and 

 west-south-west are rich in lead, those ranging north-north-west and 

 south-south-east are often unproductive. 



4. The same veins vary in respect of the nature of their contents ; 

 norne yielding lead or copper, and others copper or tin, according to 

 depth from the surface, the nature of the inclosing rock, and other 

 lew known conditions. Below the surface, 100 or more yards, veins 

 may yield principally sulphuret of lead, with a variable admixture of 

 the double sulphuret of copper and iron, and near the surface these 

 may be exchanged for carbonate, phosphate, and arseniates of lead, 

 carbonates of copper, and red oxide of iron. We may believe these 

 metallic salts to be derived from the sulphurets by processes of change 

 originating from the surface. Again, veins which cross different sorts 

 of rocks, a limestone, sandstone, and argillaceous shale, may be very 

 rich in limestone, very poor in shale, and of variable value in sand- 

 stone. Now, as the effect of dislocations in such countries is frequently 

 to cause, on the two sides of a vein, very different beds to be ou the 

 name level, so that limestone is opposite to shale, or to sandstone, or 

 to the same or a different bed of limestone, the complexity of the 

 phenomena met with in practical tniuing, even in one vein, need not 

 urprii* a prudent reasoner. Generally speaking the miner looks for 



a change in the quality of the vein with every marked change of the 

 including ground (or ' country," as it is termed in Cornwall). 



" In the older rocks we see the same vein intersecting clay-slate 

 and granite ; it is itself continuous, and there is no doubt of its 

 identity ; and yet the contents of the part inclosed by one rock shall 

 differ very much from what is found in the other. In Cornwall, a 

 vein that has been productive of copper-ore in the clay-slate, passing 

 into the granite becomes richer, or, what is more remarkable, furnished 

 ores of the same metal differently mineralised. If we pursue it 

 farther into the granite, the produce of metal frequently is found to 

 diminish." (Taylor, in ' Report to the British Association,' 1833.) 



5. This dependence of the productiveness of veins on some quality 

 of the rocks which they traverse, is a phenomenon of the same order 

 as the relation of veins in general to particular classes of strata or 

 particular masses of igneous rocks. Dislocations of the strata occur 

 in almost every district, yet it is chiefly in certain assemblages of the 

 strata that metallic veins are abundant. In Great Britain and Ireland, 

 generally speaking, the only districts of mineral veins are situated 

 among the ancient strata ; perhaps no metallic veins occur in these 

 islands above the Palaeozoic Strata [PAL/EOZOIC SERIES] ; and though 

 in other parts of the world strata of much more recent origin do 

 yield some valuable ores, it is under peculiar and limited conditions. 



6. Among these conditions appears to be the proximity of rocks of 

 igneous origin. Thus, in the Pyrenees, rocks of the age of the chalk 

 yield iron-ores near igneous rocks, and not elsewhere. On a general 

 survey of mining district?, without regard to their age, some relation 

 of this kind appears. It is near the granites of Cornwall and Brittany, 

 near the porphyries of Caldbeck Fells in Cumberland, near the green- 

 stone of Aldstone Moor, that metallic veins are abundant ; but ou the 

 other hand there is no metallic vein known in connection with the 

 sienites of Malvern or Charuwood, while the rich lead-mines of Flint- 

 shire and Grassington Moor in Yorkshire are not accompanied by traps 

 visible at the surface, or known in the mines ; and the toadstoue of 

 Derbyshire has been thought to actually cut off the lead-veins. 

 Perhaps ou this important point prudence will be best satisfied by 

 admitting the real influence of certain igneous rocks on the productive- 

 ness of mineral veins, not merely as being of igueous origin, but as 

 being of particular chemical qualities, and haviug certain definite pro- 

 perties iu relation to the passage of thermic and electric currents. 



7. A certain relation of veins to the physical geography of the 

 country is traceable. It is chiefly in or near to mountainous regions 

 that mineral veins abound ; aud this appears to be mainly owing to 

 the fact that, in such countries, the strata are more broken than else- 

 where, and more divided by masses aud dykes of igneous rocks, 

 circumstances already known to influence the occurrence of mineral 

 veins ; but several authors mistake the nature of this relation, and in 

 particular Werner, whose treatise 'On Veins' is very valuable, attaches 

 too much importance to it. He says, " The occurrence of veins 

 depends much upon the external form of the mountains, on the posi- 

 tion of the whole chain of mountains in respect to its extent aud 

 declivity, on the particular position of the country where they [the 

 mountains] occur, whether the country be composed of hills with 

 gentle declivities and roundish or flattish summits, or whether it be a 

 place in a principal valley." (Werner, ' On Veins,' p. 54.) 



Before a satisfactory theory of veins can be possible, we must 

 possess correct general iuferences on two points the origin of fissures 

 and cavities in rocks, and the nature of the forces by which those 

 fissures have been filled. Each of these points has been thought 

 difficult of approach: the former has certainly been reached; the 

 way to the latter is perhaps only indistinctly perceived. 



Fissures and other cavities in rocks exist as effects of several natural 

 causes, aud present diverse appearances characteristic of these. 



Caverns in limestone rocks are sometimes independent of real 

 fissures, and other evidence of movements of the rocks, and may even 

 be thought in some cases to be original, or left such when the rock 

 was formed in the sea. Such cavities actually occur in modern coral 

 reefs, left by the peculiar growth of the lithophytes : in certain cases 

 (for instance magnesian limestone) the small cavities may have been 

 formed by gas extricated in the course of the formation of the atoue. 

 Now in such closed cavities, large or small, the crystallisations which 

 we find (carbonate of lime, sulphate of barytes, sulphate of strontiau, 

 quartz, oxide of iron, sulphuret of iron, &c.) are of the same kinds, 

 and may be due to the same causes, as the crystallisations in originally 

 hollow shells, or in the cavities left by the dissolution of these from 

 the rocks. To the above catalogue may therefore be added sulphuret 

 of zinc, arscniuret of iron, sulphuret of lead ; for these minerals occur 

 in the situations just named, especially near faults pr^vems. 



Cracks i 

 those above-named, 



liferous fissures, lintu ni<ji* oiiin.. j / t 



carbonate of copper. (In Magnesian Limestone of Yorkshire.) 



Joints are open cracks traversing beds of stone, and come under 

 the same description as other often very large open fissures unaccom- 

 panied by dislocation. These fissures are often arranged with so much 

 symmetry as to leave no doubt of the influence of polarities among 

 the molecules of the rocks when they were formed. Joints and 

 fissures, especially near veins, often partake of the minerals which 

 they contain. 



