90 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
that, where carried to its extremes, 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 associated, or forming larger masses, of the most indefinite 
shapes, and merging into each other. Sometimes portions of partially altered preh- 
nite occur. In places, considerable 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 amyg- 
daloids, containing amygdules of prehnite, chlorite, calcite, or quartz, with more or 
less copper; other portions are in the condition described above. 
This, too, (I. and III.), appears to have been the principal period of concentra- 
tion of the copper. In the still amygdaloidal portions, this metal 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 crystallized free, where 
it had a chance: more generally it replaced other minerals on a considerable scale. It 
formed, in calcite bodies, those irregular, solid, branching forms, that are locally 
known as horn-copper, often many hundred pounds in weight; in the epidote, quartz, 
and prehnite bodies, it occurs as thread and flake-like impregnations; in the foliaceous 
lenticular chloritie bodies, it formed flakes between the cleavage-planes and oblique 
joints, or in places—and this is more particularly true of the fissure-veins, which we 
are not now considering—it replaces the chloritic, selvage-like substance till it forms 
literally pseudomorphs, sometimes several hundred tons in weight. 
When the amygdaloid has arrived at the condition we have been describing, it 
assumes some of the characters of a vein, in that, although it presents no open fissure, 
it contains greater or smaller masses of calcite and other minerals that are easily re- 
placed by an intruder. To this period, probably, belongs the replacement of calcite 
by datolite; and here, also, the rather rare occurrence of analcite crystals, and the 
pseudomorphs of orthoclase after these. 
As I have already remarked, the pseudo-amygdaloids are merely altered forms 
of the same rock as the lower zone. There seems to be a definite limit at which this 
progressive change stops, and that is when all augite is changed to its green pseudo- 
morph, and a large percentage of the rest of the rock consists of pseudo-amygdules of 
delessite, and partial pseudomorphs of this after plagioclase. The occurrence of epi- 
dote and quartz is not general, and is then confined to scattering pseudo-amygdules, — 
in which these minerals have succeeded prehnite, perhaps in the local absence of the 
conditions necessary to produce the usual delessite. 
Thus I conceive that the extent of the change to the pseudo-amygdaloidal form 
is conditioned essentially by the amount of augite present, to supply first the lime 
necessary to aid in changing the plagioclase to prehnite, and next the iron and mag- 
nesia to form the delessite, whether by acting directly on the feldspar substance or on 
the prehnite. 
The amygdaloids proper were, probably, both structurally and chemically, some- 
what different from the lower zone, in that it is reasonable to suppose that, in addition 
to being more or less porous, they contained a greater or less amount of amorphous 
base, which is more easily altered than a crystalline aggregate. But, from whatever 
cause, the amygdaloids have, as we have seen, been capable of much greater changes 
