S52 



On Veins. 



of view, in consequeoce of the curious information witli which it 

 furnishes us. The rocks are occasionally intersected by matter 

 of a diiferent kiud, which passes thrciigh them in a vertical 

 direction, or nearly so. These intersections are called veins. 

 We shall have a tolerably precise notion of the appearance of a 

 vein^ if we suppose that the rock or mountain in which it occurs 

 was by some means or other cleft in two fronn top to bottoni, and 

 that the rift has been afterwards filled- up with stony matter. 

 This stony viatter constitutes the vein. When the rock in which 

 the vein occurs is stratified^ the veins always cut through the 

 strata. 



By far the best account of veins hitherto given to the public 

 is contained in a treatise on the subject published by Werner, of 

 Freyberg. It was not drawn up, as he himself informs us, till 

 he had himself examined several thousand veins, till he iiad seen 

 specimens from several thousand more, and till he had analysed 

 all the correct accounts of the structure of veins, in every coun- 

 try which it was possible to procure. This book, it must be 

 confessed, is neither elegantly vvritteo nor well arranged; but it 

 contains a vast collection of accurate and important facts. There 

 is both a French and an English translation^ the last of which is 

 the best. 



I shall state here a few of the leading facts respecting veins 

 that appear completely established. 



1. Veins occur in every species of rock, and in every forma- 

 tion; unless some of the alluvial formations be considered as 

 exceptions : but they diminish both in variety and injportance, 

 according to the order of the formations. 



2. They vary in thickness, from 18 feet, which is the thickest 

 vein that has hitherto been accurately described, to the I2th part 

 of an inch, or even less. Their most common thickness is less 

 than six feet, and they seldom exceed that standard, 



3. They are usually widest above, and become gradually 

 narrower belov/, precisely as if they had been originally rents in 

 the rock in which they occur: and the same vein often varies 

 considerably in its thickness in different parts of its course. 



4. Somedmes veins are filled up with one single kind of stony 

 matter. Thus we have veins composed of quartz, of limestone^ 

 of felspar, of greenstone, &c. But it is common to find a- 

 variety of substances in the same vein. Thus we find limestone, 

 sulphate of barytes, and grey copper ore, in the vein at Air- 

 threy, near Stirling. In some veins more than 20 different 

 substances occur together. 



5. Wiien the veins are narrow it is more common to find 

 them of one substance ; but when they are wide, various sub- 

 stances usually occur together in them. Now the structure ot 

 these complicaled veins, if I may so term them^ deserves particu- 



