VEIN. 



rather than veins, are moft appareiit in the calyx of many 



plants. 



Veins, Metallic and Mineral, in Geology, are fiffures inter- 

 fering rocks or ilrata, filled more or lefs completely with 

 mineral or metallic matter, different from the fubftance of 

 the rock. When veins are fcen on the furface interfefting or 

 traverfing a mountain, they have heen fuppofed to refemble 

 the veins of animals ; but the refemblance is only fuperficial, 

 for veins are not tubular, except in a few inftances ; but 

 their thicknefs is fmall, compared with their length and 

 depth. 



Metallic veins are the principal repofitories of moft of the 

 metals, except iron and manganefe, which occur more fre- 

 quently and abundantly in beds than in veins. The thick- 

 nefs of metallic veins varies from a few inches to feveral feet 

 or yards : the fame vein varies alfo in thicknefs in different 

 parts of its courfe, fomctimes contracting to a narrow firing 

 of ore, and then expanding again to the width of feveral 

 yards. The depth to which they defcend is unknown, for 

 we believe no inftance has occurred of a confiderable vein 

 leing tvorked out in depth, though it may fink too deep to ren- 

 der the operation of the miner profitable ; or it may branch 

 off in a number of ftriiigs, which are too much intermixed 

 with the rock to be worked to advantage. In cafes where 

 the metallic ores have difappeared at confiderable depths, the 

 veins are ftill continued, though they are tilled excluiively 

 with the mineral matter or vein-fione which accompanied the 

 ore in the upper part of the rock. Some veins appear to 

 grow wider, and others to contraft as they defcend. The 

 direftion of veins downwards inclines more or lefs from the 

 perpendicular ; but they fometimes run for a certain diftance 

 parallel with the dip of the beds or ftrata in a mountain, and 

 then ftrike down through the lower beds. 



The length of metallic veins has rarely, if ever, been ac- 

 curately determined ; they have frequently been traced fe- 

 veral miles, but their further progrefs has been concealed by 

 the intervention of valleys, rivers, or accumulations of fand 

 and alluvial depofits. Some of the metallic veins in South 

 America have been traced to the diflance of eighty miles. 

 Large veins generally take a nearly direft line through a 

 country, except where they are turned afide by crofs veins, 

 or what are called in Cornwall crofs courfes : it is alfo re- 

 markable, that the metalliferous veins in England generally 

 run nearly eaft and weft, and the crofs courfes north and 

 fouth. To what caufe this is owing we are perfeftly igno- 

 rant. Large metalliferous veins frequently fend off fmaller 

 veins, or ftrings of ore, from their fides, which penetrate the 

 rock to a confiderable diftance on each fide of the large 

 vein. Veins are leldom entirely filled with ore, but fome- 

 times it extends in a compaft mafs from one fide to the 

 other. More frequently, the ore is intermixed with mineral 

 matter called vein-ftone, matrix or gangue : this, according to 

 the rock which it interfefts, will be either calcareous fpar, 

 iluor fpar, barytes or quartz. The vein-ftone and the ore 

 are frequently arranged over each other, hning the fides of 

 the vein with alternate layers of metallic and mineral matter, 

 and filling up the whole vein. In the mines of Cornwall, 

 the ores of copper and tin commonly occur in detached 

 maffes, which are called bunches of ore ; and the other parts 

 of the vein, being unproduftive, are called deads. 



The vein is generally feparated from the rock which it in- 

 terfefts by a thin layer of mineral matter diftinft from the 

 vein, and from the rock itfelf, and alfo by a thin hning of 

 clay. Sometimes there are large cavities in veins called 

 drufes, which are generally lined with cryftals. In other in- 

 ftances the vein divides, ijiclofing a piece of rock, which is 

 called the rider ; but it is obferved, that the inclofed mafs, 



or rider, differs in its quahty from that of the rock through 

 which the vein paffes. 



The fuperficial part of a vein gener^ly contains the ore 

 ill a decompofing ftate ; and it frequently happens that the 

 ores in the upper and lower part of the vein are different : 

 thus in Cornwall, blende, or the fulphuret of zinc, often oc- 

 cupies the uppernioft part of the vein, to which fucceeds 

 tin-ftone, and at a greater depth, copper pyrites. See ZiNC, 

 Ti's-Stone, and Copper. 



When Mr. Pryce wrote his " Mineralogia Coriiubienfis," 

 the mines of Cornwall had not been worked to a great depth, 

 for he fays the richcft ftate of a mine for copper was from 

 eighty to one hundred yards deep, and for tin, from forty to 

 one hundred and twenty yards. This account by no means 

 correfponds with the prefent ftate of the Cornifh mines. The 

 Dolcooth copper-mine, near Redruth, is worked to the depth 

 of four hundred and fifty-fix yards, and is very productive 

 at that depth. 



Veins generally decline from the perpendicular, and de- 

 fcend into the earth obliquely. The fides, or, as they are 

 called, the walls or cheeks of the vein, are differently deno- 

 minated, the upper fide being called the hanging-fide, the up- 

 cheek, or hanger ; and the under fide, the hading-fide, the 

 dotun-cheek, or the ledger. The veins we have been de- 

 fcribing are called rake veins in fome parts of England, and 

 in Cornwall, they are denominated lodes ; which fee. Thefe 

 metalliferous veins have commonly the fame direftioii, or 

 nearly fo, in the fame diftrift, and the veins which crofs them 

 are generally unprodudtive, or contain metallic ores of a dif- 

 ferent kind. They are called crofs courfes, or north and 

 fouth veins. 



Metalliferous rake-veins interfeft moft of the mountains 

 called primary, fuch as granite, gneifs, and mica-flate. 

 (See Granite, &c.) But they are more abundant in flate- 

 rocks than in rocks of granite or porphyry. ( See Granite, 

 Slate, and Porphyry.) They alfo interfeft the rocks of 

 tranfition and mountain lime-ftone, which reft upon flate, or 

 altei-nate with it ; but they rarely rife into the fecondary 

 ftrata which contains coal. This faft feems to prove that 

 veins were formed prior to the depofition of the upper fe- 

 condary ftrata. When a metallic vein in its defcent paffes 

 through different kinds of rock, it is frequently obferved 

 that the produfts of the vein vary in each bed ; and when it 

 paffes through regularly ftratified beds of the fame rock, 

 there are particular ftrata in which the veins are always 

 found moil produftive, and thefe in the north of England 

 are called bearing mtafures. If the nature of the rock ieems 

 to have produced a change in the quality of the ore, it is 

 no lefs remarkable that the rock itfelf is alfo frequently 

 changed or decompofed in the immediate vicinity of a vein. 

 This change is more apparent in fome rocks than in others, 

 particularly in granite, fienite, gneifs, mica-flate, argillaceous 

 fchiftus or flate, and porphyry. In fuch inftances, accord- 

 ing to Werner, it is only one of the component parts of the 

 rock that is decompofed, either the felfpar, the hornblende, 

 or the mica, but never the quartz. This change fometimes 

 extends to a confiderable diftance on each fide of the vein, even 

 to a fathom or more ; it extends farther in fome places than in 

 others, and is moft general in thofe parts where the vein con- 

 tains fulphur. Sometimes this change in the rock may be 

 perceived to far, that it ferves as a guide to the miner ; and 

 in following a fterile vein, when he comes to a place where 

 the rock is decompofed, he concludes that the metalhc ore 

 will foon be found. In Cornwall, the felfpar is frequently 

 changed in the vicinity of a vein, and tin-ftone is fometimes 

 diffeminated through the rock to fome diftance on each fide 

 of it. 



The 



