CUPRIFEROUS CONGLOMERATES AND AMYGDALOIDS. 421 
In these bowlders the copper has replaced both the matrix and the porphy- 
ritic feldspars, occurring in the latter, when the replacement has not been 
carried very far, often along the cleavage lines only. Pumpelly has shown 
that the deposition of this copper has always followed other great changes 
in the condition of the porphyry fragments, and notably the replacement 
of both matrix and feldspars by chlorite and epidote; these minerals hay- 
ing in turn been replaced by the copper. This relation, between copper, 
epidote and chlorite, is one which exists also in the altered amygdaloids; 
and the source of the constituents of these minerals may be found either in 
the particles of amygdaloid matrix and other basic materials which not 
unfrequently occur in the conglomerates themselves—in the Nonesuch sand- 
stone forming a predominating quantity—or in the overlying trappean beds, 
from which they may have descended along with the infiltrating carbon- 
ated waters. 
The ordinary cupriferous amygdaloids, such as those which are so largely 
mined about Portage Lake, are, as Pumpelly was the first to show, simply 
the more or less completely altered and copper-saturated upper vesicular 
portions of the old lava flows, and are neither independent layers, nor 
“veins” parallel with the formation. The copper has been introduced into 
these amygdaloids during one of the later stages of a long chain of replace- 
ments, whose history has already been briefly outlined, as worked out by 
Pumpelly, on a previous page. Several paragraphs of his descriptions may 
appropriately be quoted again in the present connection. 
Considerable portions of the bed have lost every semblance of an amygdaloid, 
and consist now of chlorite, epidote, calcite, and quartz, more or less intimately asso- 
ciated, or forming larger masses, of the most indefinite shapes, and merging into each 
other. Sometimes portions of partially altered prehnite occur. In places, considera- 
ble masses of rich brown and green fresh prehnite filled with copper occur; but, as a 
rule, this mineral has given way to its products. 
To this process the copper-bearing beds of Portage Lake—wrongly called lodes 
—owe their origin. Considerable portions of these beds are but partially altered 
amygdaloids, containing amygdules of prehnite, chlorite, calcite or quartz, with more 
or less copper; other portions are in the condition described above. 
In the still amygdaloidal portions, the copper was deposited in the cavities and 
in cleavage-planes of some minerals, and replaced calcite amygdules, ete. But in the 
confused and highly altered parts of the bed it erystalized free, where it had a chance; 
more generally it replaced other minerals on a considerable scale. It formed, in 
