LEAD AND SILVER. 



197 



with more or less strongly marked fissures which formed the con- 

 duits. Mr. Curtis made a careful series of assays of the neigh- 

 boring igneous rocks to find some indication of the source of the 

 ore. A quartz porphyry gave significant results and to this the 

 metals are referred, but the portions of the mass at a great 



FIG. 50. Section at Eureka, Nev. Reproduced in line work after colored 

 plate by J. S. Curtis, Monograph VI., U. S. Geol. Survey. 



depth are considered to have furnished them. " Eureka was one of 

 the first places in this country where the hypothesis of. replace- 

 ment was applied to ores in limestone. The district is now far 

 less productive than it was ten or fifteen years ago. 1 



1 G. F. Becker, Tenth Census, Vol. XIII., p. 32. Rec. W. P. Blake, 

 " The Ore Deposits of the Eureka District, Nevada," M. E., VI. 554. J. S. 

 Curtis, "Silver-lead Ore Deposits of Eureka, Nev,," Monograph VII. 9 



