The Tlicory of Copper Deposition — Lauc. 309 
Xatiiiiial iniiu', Ixit in i^xMirral sulphates arc rare. 'I'lic arsen- 
ides and sulphides that dn dcctn- are very peculiar, occurring 
mainly in the veins, and perhaps rather more frequently as at 
nuiunt ])oheniia, associated with the acid rocks. Idierc are 
si^ns that at least at times they are secondary after the native 
copper. It has occurred to me that possibly a ferrous or ferric 
chloride containimj^ a trace of copper was an early volcanic 
emanantion. It is, however, also true that olivine, which is 
one of the earliest minerals to develop, contains ferrous silicate 
with which is likely to be associated a trace of copjjer and 
nickel. I'urthermore, under the microscope the olivine, an early 
formed mineral, appears to gather at the sides of this dike 
and the top of the flow. Analyses (Vol. VI. and here) seem 
to indicate the same thino- in the variation of the magnesia 
and iron. 
Thus the copper may have been concentrated, l-'irst, with 
the olivine of the amygdaloid traps; secondly, by leaching out 
of the olivine which decomposed either by atmos]:)]ieric action 
and meteoric waters, or immediately after the outflow of the 
lava in the presence of the w^aters, acid and ])erhai)S hot. buried 
with this formation ; thirdly, by reactions due to the circulation 
downward of this water set up by this uplifting of the edge 
of the great lake Superior synclinal. It must also be remem- 
bered that according to the earlier geologists there has been 
enormous erosion, which, according to L. L. Hubbard's theory 
(VI., p. 94), may lie in part replaced by a sliding or the upper 
beds on the lower for miles. In either case there may have 
been a considerable migration downward, in the porous belts 
of the formation, of the material of the strata and the original 
water thereof. 
There is yet much to be learned. Init three things appear to 
me to be extremely probable ; the copper was associated with 
the original lava flows; that originally deposited water or gas 
has been an important factor, possibly merely in bringing cop- 
per into solution, and that the water circulation which finally 
precipitated the copper w^as downward. 
It is apparent, however, that we need to test the rival theo- 
ries. We need to trace some one horizon some one conglomer- 
ate or flow continuously through and survey it carefully and 
accurately to detennine the minor flexures. Dr. L. L. Hub- 
bard has done this in jiart. but the work is not complete. 
