456 ORVILLE A. DERBY 
The conditions above enumerated are, in their most essential 
particulars, strikingly similar to those under which the mineraliza- 
tion, with auriferous sulphides, took place in the Passagem lode, as 
set forth in my recent paper in the American Journal of Science 
(September, ro1r), in which, however, no mention was made of two 
circumstances of certain importance for the present discussion. 
These are: (1) large and ideally perfect crystals of arsenopyrite 
occur in certain portions of the hanging wall under conditions cor- 
responding exactly to the cases of replacement cited by Irving for 
the sulphides pyrite and galena,’ and (2) slick-sided planes at or 
near the upper side of the lode are abundantly coated with graphite 
which is at times segregated in lumps of the size of the fist. 
Whether in this case the carbon separated out from the 
tourmaline-forming or the sulphide-forming solution could not be 
determined and is immaterial in the present discussion. Whatever 
its carrier may have been, it is here certain that carbon was 
deposited in one of its two solid mineral forms from a solution 
capable of dissolving portions of the rock in which it circulated, 
and thus of opening space for the deposit from some of the other 
of its mineral contents, in the form of replacement minerals such 
as tourmaline and arsenopyrite. 
«The containing rock consists of a finely granular mixture of quartz, magnetic 
pyrite, and calcite. Where replaced, the two latter minerals disappeared while the 
quartz granules remained locked up in the crystals of arsenopyrite. 
