440 



MINERALOGY. 



Geognosy. Veins composed of mountain-rocks sometimes pre- 

 sent a stratified appearance. There are many inter- 

 esting examples of this fact in the island of Arran. In 

 that uncommonly interesting spot, wacke and green- 

 stone occur in layers in the same vein. In other cases, 

 veins are composed of layers of greenstone, porphyry- 

 slate, and pitchstone. Other kinds of stratified veins 

 occur in Arran and other parts of Scotland, which we 

 shall take another opportunity of describing particular- 



Open 

 paces ex- 

 tending 

 from side 

 to side of 

 the vein. 



Intersec- 

 tion! of 

 reins. 



Old and 

 new veins. 



Change of 

 direction. 



Different 

 groups of 

 minerals or 

 formations 

 in the same 



Metallifer- 

 ous veins 

 run paral- 

 lel with 

 great ral- 

 Iff* 



6. Besides the drusy cavities, there sometimes occur 

 very considerable open spaces in veins, that reach from 

 the one side to the other, or from the hanging to the 

 lying side. 



7. When we attend to the masses of which veins- are 

 composed at different depths, we observe that they 

 approach nearer and nearer to the centre, towards the 

 lower part of the vein, but expand or recede from it 

 in the upper part. Thus, those layers, which in the 

 upper part of the vein are near to its sides, are at a con- 

 siderable distance from them lower down, and, still 

 deeper, approach nearer to the middle of the vein. 



8. The appearances produced by the meeting, and 

 intersecting of veins, are highly curious and important. 



a. When a number of veins, that do not intersect 

 one another, occur in the same district, it is observed 

 that they have usually the same direction and consti- 

 tuent parts ; thus intimating, that all of them are of the 

 same general formation. When different venigenous 

 depositions occur in the same district, then the direc- 

 tion of the veins of these different depositions is va- 

 rious, and they are arranged in such a manner, that 

 each particular formation has a certain direction. 



6. Veins running in different directions, either cross, 

 or simply meet one another. When they cross, that 

 which is crossed or intersected is said to be the oldest, 

 and that which crosses or intersects, the newest, an opi- 

 nion, the accuracy of which may be questioned. 



c. Veins sometimes cross each other, without cau- 

 sing any change of direction ; but more frequently we 

 find the direction considerably changed. 



d. When veins meet under an acute angle, the new- 

 er frequently traverses the older ; runs parallel with it 

 to a considerable extent, on its lower side, and then 

 again diverges under the same angle it crossed it ; 

 sometimes the newer vein does not fully traverse the 

 older, but changes its direction in the middle of the 

 older vein ; runs through the body of the vein ; and 

 after a longer or shorter course, again diverges at the 

 same angle it entered. Sometimes the newer vein does 

 not even traverse the older; only meets it ; then runs 

 parallel with it; again diverges; and this is sometimes 

 frequently repeated in the course of the vein. 



Sometimes newer veins do not even run parallel 

 with the older, but fairly terminate in them ; and this 

 takes place usually on the hanging or upper side. 



9. In the same veins, we sometimes meet with two 

 or three different formations. Thus, in some Saxon 

 metalliferous veins, we find the lowest formation to be 

 lead-glance ; immediately above it, a formation of na- 

 tive silver ; and the uppermost sparry-ironstone. In 

 France, there are veins, whose lowest formation is cop- 

 per-ores ; immediately above it, a formation of silver- 

 ore ; and the uppermost is a formation of iron-ore. 

 Many more examples of the same kind might be men- 

 tioned. 



10. Great metalliferous veins usually run parallel 

 with the general direction of great valleys. 



Geognosy 



Veins 

 niore fre- 

 quent in 

 hilly thai. 

 in moun- 

 tainous 

 country. 



Mode of 



formation. 



11. Veins also occur more frequently in flat hilly 

 country, than in steep mountainous country, and ge- 

 nerally on the ridges of the hills. Beds, on the contra- 

 ry, are equally, if not more abundant, in steep moun- 

 tainous country. 



12. Sometimes the strata or beds, traversed by veins, 

 are merely separated ; so that the strata or beds on op- 

 posite sides of the vein correspond. In other cases, 

 there is a dislocation, or what miners call a shift of the 

 strata, that is the similar strata or beds on opposite 

 sides of the vein, do not correspond, but are depressed 

 or sunk generally on the hanging or upper side of the 

 vein. The degree of depression or dislocation usually 

 corresponds to the magnitude of the vein. 



13. The walls of veins are frequently more or less 

 altered ; and this alteration is caused either by an in- 

 termixture of the materials of the vein with that of the 

 wall, or by a decomposition of the wall, owing to the 

 agency of percolating water, or the substances of which 

 the vein is composed. 



Mode offormation. Veins are either of simultaneous 

 formation with the rock which they traverse, as in gra- 

 nite, metalliferous veins, &c. or are rents that have been 

 filled up from above with the mineral matter they 

 now contain, as in those veins that contain true frag- 

 merits. 



The various economical relations of veins will be 

 discussed in the article VEIN. 



%* AOE OF METALS. 



Age of metals. It appears from the details already Age of 



given in our account of mountain-rocks. metal*. 



1st, That metals differ very much as to the period of 

 their formation. 



2</, That the variety and quantity of metalliferous sub- 

 stances decrease in general from the primitive to the 

 alluvial period of the earth's formation. 



3d, That molybdena, titanium, tin, scheele, cerium, 

 tantalum, uran, chrome, and bismuth, are metals of 

 the oldest primitive formation, and that only feeble 

 traces of them are to be observed in newer periods. 



bth, That although arsenic, cobalt, nickel, silver, and 

 copper, occur in old primitive mountains, they also 

 extend to newer mountains. 



5th, That gold, tellurium, antimony, and manganese, 

 are metals of a middle age, occurring in the newer 

 primitive, the transition, and the oldest secondary- 

 rocks. 



6lh, That lead, zinc, and mercury, are of later date, 

 when compared with those metals we have already 

 mentioned, because they occur in greatest quan- 

 tity in the newer or secondary formations. 



"7th, That iron is found in every rock, from the oldest 

 granite to the newest alluvial deposit ; hence is uni- 

 versally distributed, and is therefore a production of 

 every period. 



8lh, That the more crystalline ores abound in the pri- 

 mitive mountains, but continue decreasing in quan- 

 tity and variety from the primitive rocks to the new- 

 est alluvial deposits. 



*,* THEORY OF THE FORMATION OF THE EARTH. 



This curious but very extensive subject cannot be Theory of 

 discussed in the present article, but will be considered the r rn - 

 in all its bearings, and in connection with such new ^ r " t f ' 

 facts and views as may occur to travellers and geolo- 

 gists, or may result from our own investigations, un- 

 der the article- THEORY OF THE EARTH. 



