REVIEWS. 



355 



Alston Moor, In these he arjnics, that as lead ore is not deposited t1iroiii;h- 

 out tlie wliolc extent of the veins, they have not always been liilcd wlik 

 minerals oi- metallic substances; the deposition must theref(n'e he 1 he result 

 of certain antecedents ; and he infers that the laws w liic-hhave rc'^uhilc-d the 

 disXribution of metallic ores may be different from those relatin;^- to the 

 ori^nnation of the metal. Two theories have been proposed to account for 

 the ori<i;in of ore-veins : the one supposes them to be segregations of me- 

 tallic particles from the surrounding; rocks, the other regards them as de- 

 posits by sublimation from great depths and connected with volcanic in- 

 fluences. It is certain, however, that the la\^ s which have r.'^iihilcil I lie 

 distribution of ore in veins may be of a very different chaiacter from Ih^so 

 connected with its origination ; the former may be mechanical, the latter, 

 if metals are substances compounded from certain elements unknown in a 

 separate state, must be chemical. If they are simple substances \^'hi( li 

 have risen from the interior of the earth as gaseous emanations, then the 

 accumulation of the ore in certain portions of the veins may have taken 

 place after its deposition sparsely throughout the whole extent of the frac- 

 tures in the rocks ; if the metals are derived from rocks in which their ex- 

 isLonce cannot be detected, then the compounding and localization of the 

 ore may have been effected contemporaneously. The experiments of Bcc- 

 querel and others have shown that metals in solution may be crystallized 

 and combined with other substances by electro-chemical agency, fonning 

 minerals exactly similar to those found in nature ; but interesting as those 

 experiments are, they render no assistance to the practical miner in guiding 

 liim to the deposits of metallic ores so irregularly distributed in the veins ; 

 they relate more to crystallography and mineralogy than to practical 

 mining, for crystals of various kinds are found where none of the metnls 

 exist. No kind of mineral in the veins at Alston Moor varies so much in 

 quantity in different parts of the same stratum as lead-ore. It is found 

 plentifully deposited with quartz, carbonates and sulphides of lime and 

 iron, fluorspar, barytes, oxides of iron, black-jack, etc., and it is frequently 

 absent in the same stratum when the veins contain large quantities of some 

 one or other of these minerals ; hence its deposition is not dependent on 

 the presence or absence of any of those minerals, and it is evident that 

 whatever caused llu ir (h^position has not prevented the operation of those 

 causes which regulated tlie deposit of lead-ore. The variation in amount 

 of lead-ore in the snme vein and in the same stratum or kind of rock be- 

 ing greater than that of any other mineral, the law of such variation is 

 more likely to be ascertainable in its case than in any other. In Alston 

 Moor the veins have been most productive in situations furthest removed 

 from plutonic action; the richest deposits having been effected in the 

 upper part of the mountain limestone, where no igneous rocks are found 

 either in the form of dykes, or of sheets intermingled horizontally ^^ ifh tlie 

 stratified rocks. The lower part of the strata in that district includes a 

 stratum of basaltic greenstone as well as a basaltic dyke, but the veins 

 generally have contained very little lead-ore, where these rocks have formed 

 tlieir si(h\s or walls — a circumstance, however, we should think might very 

 ])n)bably be sometimes due to the sublimation or driving oll ol'the" lead by 

 the heat of the inje(;ted lava in the cases of volcanic dykes. 



So far as the Alston ]\Ioor district is concerned, Mr. AA allace thiid<s 

 there is nothing to support the theory that the lead is due to exhalations 

 from beh)w, or to matter injected in a fluid state among the consolidated 

 sedimentary rocks. The nodiUes of carbonate of iron, so often fouiul ar- 

 ranged in layers of beds of shale, have generally undergone some degri-e 

 of conf ract ion in tlu> inlerioi"; often (lie exti-rioi- has \)vv\\ ron>o\\(\i\ii\\ \o a 



