244 Miller on the Taconic 
was, in Taconic days, represented by a volcano, which has sunk 
beneath the waters of the lake. The flows were followed by 
detrital rocks representing the intervals of time between them ; 
but these detrital rocks are composed largely of conglomerate 
layers and large sized pebbles, indicating strong currents of 
water. The flows visible upon the borders of the lakes were 
forced through fissures by volcanic energies. The copper, 
which occurs in the conglomerates, amygdaloids, epidote veins 
and otherwise, is supposed to have been precipitated from water 
holding it in solution, or leached from detrital rocks where it 
was originally deposited in a sulphureted form. R. D. Irving, 
who has studied closely the copper bearing rocks of this region, 
says the explorer for transverse veins should bear in mind that 
epidote, prehnite and chlorite are favorite associates of copper, 
while laumonitic veins and those bearing a predominating quan- 
tity of calcite are not so rich; that a wide vein in amygdaloidal 
or other soft rocks will pinch to a mere seam, \vithin the mas- 
sive and compact layers; and in sandstone and conglomerate 
deposits the valuable belts have been found where the conglom- 
erate is overlaid with trap, or in sandstone very rich in basic 
detritus. Any of the conglomerate seams from Keweenaw 
point to Minnesota may be cupriferous. All of the upper div- 
ision of the series is non-cupriferous, except the Nonesuch sand- 
stone belt in the Porcupine mountains; and all the belts and 
areas of acid rocks, such as the central area of the Porcupine 
mountains, and the great spread of red rock, in the Bruld lake 
country in Minnesota, and all belts and areas of coarse-grained 
basic rocks, such as the great area of coarse gabbro in the Bad 
river region in Wisconsin, and the similar area which occupies 
the belt of country from Duluth to Bruld lake, are also non- 
cupriferous. The slates and quartzytes of the Taconic system 
which lie below the Cupriferous series on the north shore of 
lake Superior, have been called the Animikie group. About 
three-fourths of the great thickness of the rocks is referred to 
Volcanic overflows and does not therefore, belong to the geo- 
logical column, the whole of which is the result of sedimentary 
deposition. 
The vast extent and great thickness of the rocks, resting on 
the granite and gneiss and followed by the Potsdam sandstone, 
