Joseph LeConte—Genesis of Metalliferous Veins. 19 
‘one igneous rock and another erupted at a different time, there 
will be a plane of weakness, and as such liable to be opene 
by crust movements. Every such plane of weakness will in- 
ae the copper and silver-bearing Triassic sandstones of 
mode of formation is substantially one. The study of these 
~ 
. . 
one principle. 
Berkeley, Cal., March 27, 1883. 
- * Cazin, N ewberry etc., Re iento C i i 
.. . .. Report of Nacemiento Copper Mines, New Mexico. 
_ + Schmidt, this Journal, vol. xxi, p. 502, 1881. 
