194 A POPULAR EXPOSITION OF THE 
higher beds of the upper copper-bearing series of the lake region,— 
the lower beds of this series, as explained above, being now generally 
referred to the Potsdam Group. They consist of quartzose sand- 
stones, red and greenish sandstone conglomerates, various limestones 
and shales, and interstratified masses of compact and amygdaloidal 
trap. These beds are also intersected by numerous trap or green- 
stone dykes; and a vast mass of trap, in places of a basaltic char- 
acter, generally caps the entire formation. The total thickvess of the 
group is estimated by Sir W. Logan as not far short of 10,000 feet. 
The cavities in the bedded amygdaloidal traps are filled with agates, 
amethyst-quartz, cale spar, various zeolites, green earth, epidote, 
specular iron ore, native copper, &c. Some of the intrusive dykes 
are porphyritic, and a few consist of syenite. (See Parr III). The 
greenstone dykes present everywhere a transverse columnar structure, 
and are frequently of great width. As they usually resist the disin- 
tegrating action of the water and the atmosphere better than the 
main body of the rocks which they traverse, they often stand out in 
relief and form buttress-like masses extending into the lake, so as to 
produce many natural harbors and breakwaters. The rocks of this 
series are also traversed by a considerable number of mineral veins 
belonging, according to the officers of the Survey, to two distinct 
systems, ‘some being parallel with the range of the strata, whilst 
others run in a converse direction to this. The veinstones consist 
usually of calc spar, heavy spar, or quartz; but sometimes of chert 
or agate, or of the above substances mixed with various zeolites, fluor 
spar, copper, coppet-glance, the common and purple copper pyrites, 
gaiena, and blende, in addition to much iron pyrites. The more 
important metallic sites comprise Prince’s Location (native silver and 
silver glance) ; Harrison’s Location, St. Ignace Island (native copper 
with native silver); Mamainse (native copper and copper ores) ; and 
Michipicotin Island. At the latter locality, native copper (in places 
slightly argentiferous) occurs not in a vein, but in nodules distributed 
through a bed of amygdaloidal trap. The other economic minerals 
of these rocks, include the sulphate of baryta (heavy spar) of Thun- 
der Bay ; the amethyst quartz of the same locality ; and the agates 
of Michipicoten and St. Ignace. 
Exposures of these higher beds of the upper copper-bearing series, 
occur principally on the south-east side of Thunder Bay, where they 
form an escarpment of white sandstone (the bottom rock of the higher 
group) about 200 feet high; also between Thunder Bay and Black 
s 
