PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 439 



Dr. Stevens.— The next ore in value which is found in this 

 system of rocks, is plumbago, which is found from six to twelve 

 inches in thickness. It is carbon almost pure ; and it is a ques- 

 tion among many geologists how to explain the existence of car- 

 bon in a system of rocks, which, in their opinion, never had any 

 vegetables growing upon it. Plumbago is found in Worcester, 

 Massachusetts, in the true coal system ; formed, as is supposed, by 

 the conversion of bituminous coal into anthracite, and then by 

 the metamorphed actioa being carried still further, in some places 

 converting the latter into plumbago. Another mineral, in the 

 primitive formation, is copper — the blue, black, yellow and greea 

 copper ores. 



Zinc is also a metal belonging to the primitive formation, both 

 in the form of calamine or silicate of zinc, and of carbonate of 

 Bine. The mines of Tennessee are in this formation, and I believe 

 those in Pennsylvania are on the borders of this formation. The 

 zinc of the Western States is found wholly in another formation. 

 The metals are often found associated — copper and zinc, copper 

 and iron, copper, iron and gold, so that a vein at first worked for 

 gold may pay to be worked for copper. This brings us to the 

 theory of veins. 



The Plutonian theory held that all veins were injected from 

 beneath ; that the centre of the earth is .a vast magazine of all 

 the metals, and by some of the vast forces of nature, which they 

 hardly pretended to understand themselves, gold would come up 

 in one place, iron in another, copper in another, silver in another, 

 lead in another. The inference often drawn from this theory was 

 that if a vein would pay to work upon the surface, the deeper 

 you went the nearer you would come to the central mass and the 

 richer would the vein become.* One consequence of a belief in 

 this theory has been that an immense amount of money has been 

 sunk in following out that doctrine. But I do not know that 

 there is satisfactory evidence that a single vein upon the wid» 

 earth was ever injec^ted from below. In many places in tlie Uni- 

 ted States we know it is not so, because the veins have been 

 worked entirely out. It has been thought sometimes that that 

 was because we did not go deep enough ; that there was merely 

 a break in the vein, and that if we could only reach it again it 



* This inference was mechanically incorrect, for if veins were formed by injection the 

 closest matter ought to have been projected to the greatest distance from the central 

 igneous mass. — J. B. 



