and us associates on Lake Superior. 351 
| seem that the direction was determined by the presence or re- 
| lative freedom from free carbonic acid. The deposition of cal- 
| _ cite, if formed from the acid carbonate, would set sufficient 
‘) carbonic acid to prevent the formation of silicates of iron and 
| ‘Magnesia 
and there in the less amygdaloidal melaphyr in minute specks 
* nd impregnations, or even in a more concentrated form as thin 
__ Sheets occupying the joint-cracks. 
ee These occurrences increase in frequency in proportion as the 
rock is more amygdaloidal; in other words, the copper is more 
concentrated in those portions of the beds where the chemical 
change has been greatest. Where the rock has not passed be- 
yond the strictly amygdaloidal stage, the copper occurs in the 
amygdules traversing these in flakes, or coating them ina film of 
greater or less thickness, to such an extent as toform from % per 
cent to 3 per cent by weight of the rock over considerable areas. 
Finally, in those beds where the ——— has proceeded 
such an extent as to wholly replace large portions of the 
amygdaloid by secondary minerals, epidote, calcite, quartz, 
chlorite, laumontite, etc., there the copper occurs in masses of 
many pounds, and sometimes of several tons weight, and in 
forms equalled in their irregularity only by those of the masses 
of secondary minerals accompanying the metal. ee 
In each and all of these positions we find that the deposition of 
_ the copper took place subsequently to the decomposition and 
Temoval of a portion of the rocks, and subsequently to the de- 
Position of laumontite, epidote, prehnite, and quartz, where 
nese accompany it. ; 
n all this we have direct evidence of the movement of 
Some salt of copper in wet solution, and the concentration of 
the metal by accumulating deposition in places where the preci- 
Pitating agent exis 
_ The Quebee group, to which these rocks belong, and whic 
Consist in various places of undoubted sedimentary strata ex- 
hibiting every degree of metamorphism, 1s as strongly charac- 
= by copper as the Galena limestone is by 
Except in th 
— e melaphyrs of Lake Superior, the copper, so 
widely diffused in the crab of the Quebec group, exists either 
