4 THIRD REPORT — 1833. 



The only dimension we can ascertain is that across from one 

 side to the other of the tabular mass, and is measured from one 

 wall to the other, which is the term used in England for the 

 cheeks or sides presented by the inclosing rock. This dimen- 

 sion is called the w^idth, or frequently the size of the vein. 



The width varies considerably in the same vein. In Europe 

 a vein containing ore is considered to be a wide one if it ex- 

 ceeds five or six feet. In Mexico the width of vehis is gene- 

 rally greater. 



In metalliferous veins the deposits of ore are extremely irre- 

 gular, forming masses of very diversified form and extent, and 

 are separated from each other by intervening masses of vein- 

 stone or matrix, either entirely devoid of ore, or more or less 

 mixed with it. It is rare to find a vein entirely filled with ore 

 in any part. 



In this respect they differ from most beds, where, as in those 

 of coal, the whole is a uniform mass. 



The layer of clay, which, as Mr. Carne says, is frequent in 

 such veins, will deserve particular notice when we consider 

 their general structure and the theories of their formation : 

 this is called Saal-bande by the Germans, and flookan by the 

 Cornish miners. 



The clearest idea of a vein will be obtained by imagining a 

 crack or fissure in the rocks, running in nearly a straight line, 

 extending to great and unknown length and depth, and filled 

 with various substances. 



I do not intend by this description to convey any theoretic 

 opinion as to the manner in which such fissures may have been 

 formed, or as to the mode of their being furnished with their 

 present contents. These are subjects on which the greatest 

 diversity of opinion has existed in former times, and this diver- 

 sity is continued to the present period. It is the main business 

 of this Report to state these opinions, and to describe our pre- 

 sent state of knowledge of this difficult subject. I feel great 

 distrust of my power to do it justice; but I am encouraged by 

 the idea that a feeble sketch may induce abler hands to pursue 

 the design, and throw more and more light upon this interesting 

 branch of geology. 



It would be of little use to go into details of the conjectures 

 of ancient authors, or into the mysteries with which this sub- 

 ject was enveloped in the age of alchemy. 



The earliest writer who is worthy to be consulted is Agricola 

 (whose proper name was Bauer) : he resided in the Saxon Erz- 

 gebirge, and died in the middle of the sixteenth century. He 

 has been called the father of mineralogy, and of the science of 



