and its associates on Lake Superior. 349 
visible in the holes in the altered feldspar, and the cleavage 
planes often glisten with flakes of copper. As we go further 
chloritic mineral, which whitens before the blow-pipe, and fuses 
on the edges to a grey glass. A little further from the center 
there is no longer a trace of the porphyry matrix: it is altered 
wholly to chlorite. The feldspar crystals are somewhat more 
altered here than they are in the middle of the pebble, but the 
quartz grains seem to have been in part replaced by chlorite. 
The change to chlorite is accompanied throughout by the pres- 
ence of a large amount of copper. While in the interior of the 
pebble, the flakes of copper are confined to the cleavage planes 
of tne feldspar, and the orphyry matrix exhibits scarcely a 
ce of the metal, the oe which has replaced the matrix 
contains in different parts of the specimen from 10 to 60 per 
cent, by weight, of copper. : 
tn another pebble of the same porphyry, not only is the ori- 
ginal matrix gone, but the usurping chlorite has been almost, 
if not wholly, replaced by copper; and we have as the remark- 
able result 4 quartz-porphyry, whose crystals of feldspar and 
Stains of quartz lie in a matrix of metallic copper. There is 
still a very small amount of chlorite resent, but it seems to 
have come from the change of the feldspar crystals and quartz 
and dirtier brown than in the previous cases, which may be 
due to the — of manganese in the alteration product 
t 
Zatio 
_ The entire pebble is permeated with minute —s threads 
and plates of carbonate of lime. The lighter-col portion 
©ontains considerable copper, while nearer the surface of the 
Pebble it is largely replaced by that metal. Pebbles showing 
the various alterations described above are by no means rare. 
Many of them, from 1 inch to 1 foot in diameter, are found 
every day. é 
IIL Conclusions. : 
We may be permitted to draw a few conclusions from the 
| facts brought out in the observations thrown together in the 
_ “Fegoing pages. 
