636 RECORD OF MEETINGS OF THE 



Copper, percentage beyond the fourth or fifth place of decimals, 

 that is in the hundred thousandths or millionths of one per cent. 



Lead and zinc, percentages in the fifth place of decimals, or 

 in the hundred thousandths of one per cent. 



Silver, percentage two decimal places beyond copper — or in the 

 ten millionths to the hundred millionths of one per cent., or the 

 ten thousandth to the hundred thousandth of an ounce to the ton. 



Gold, percentage one tenth as much as silver. 



Tin, percentage in the fourth or fifth decimal place, that is, 

 in the ten thousandths or hundred thousandths of one per cent. 



These figures, inconceivably small as they are, convey some 

 idea of the rarity of these metals as constituents on the average 

 of the outer six or eight miles of the earth's crust. But they 

 are locally more abundant in particular masses of eruptive 

 rocks which are associated with ore deposits. 



In the following tabulation I have endeavored to bring 

 together a number of determinations which have been made 

 in connection with investigations of American mining districts. 

 In a general way they give a fair idea of the metallic contents 

 of certain eruptive rocks from which were taken samples as 

 little as possible open to the suspicion that they had been 

 enriched by the same processes which had produced the neigh- 

 boring ore-bodies. 



Copper, 



Lead, 



Lead, 



Lead, 



Zinc, 



Zinc, 



Silver, 



Silver, 



Silver, 



Gold, 



Gold, 



1 Average of eight eruptives from Missouri, Anal, by J. D. Robertson. 

 Report on Lead and Zinc, Mo. Geol. Surv., II., 479. 



2 Average of six different rocks, embracing eighteen assays; S. F. 

 Emmons, Monograph XII., U. S. Geol. Surv., 591. . 



3 One rock, a quartz porphyry, not certain the rock was not enriched. 

 J. D. Curtis, U. S. Geol. Surv., Mono. VII., 136. 



4 Same reference as under 6. The zinc was determined in but two 

 samples. 



s Same reference as under 6, but p. 594. 



« S. F. Emmons, XVII. Ann. Rep. U. S. Geol. Survey, Part II., p. 471. 



1 A. Simundi in Tenth Census, XIII., 54. 



.009 



Missouri. ' 



.0011 



Colorado. 2 



.008 



Eureka, Nev. 3 



.004 



Missouri. 1 



.0048 



Leadville, Colo. 4 



.009 



Missouri. 1 



.00007 



Leadville, Colo. 5 



.00016 



Eureka, Nev. 3 



.00016 



Rosita, Colo. 6 



.00002 



Eureka, Nev. 3 



.00004 



Owyhee County, Id. 7 



