EXPLORING FOR COPPER. 427 
amygdaloid beds with the favorable veinstone, even though it show only a 
little copper at surface, is worthy of examination. 
Should our explorer be looking for cupriferous belts, he should see that 
they are well defined ; that they present evidence of much alteration such 
as is above indicated; and that one or more of the favorite associate min- 
erals of the copper are present. These favorable indications, along with a 
more or less well preserved amygdaloidal character to the rock, and the 
presence of some copper at surface, are sufficient to warrant further exam- 
ination. In searching for these belts care should be taken not to be misled 
by the occurrence of seams of native copper without veinstone along the 
joint cracks of an unaltered massive diabase, and of isolated pockets of 
epidotic and calcitic material carrying some copper. 
In the case of sandstone and conglomerate deposits the explorer is to 
bear in mind that thus far they have been found only where a thin seam of 
conglomerate is directly overlain by a trappean mass; or if away altogether 
from the trappean beds, only in sandstone which is very rich in basic det- 
ritus. Beyond this, there is nothing to guide him except the finding of the 
copper itself. Any one of the numerous conglomerate seams which from 
Keweenaw Point to Minnesota are everywhere interbedded with the pre- 
vailing basic flows, might become cupriferous at any point along its course. 
Large portions of the Keweenaw Series may be thrown out of the ques- 
tion in considering the possibilities of future discovery of copper in the Lake 
Superior region. Thus the whole extent of country occupied by the Upper 
Division of the series, with the one exception of the Nonesuch sandstone 
belt, appears to be non-cupriferous. The extent of the Upper Division is 
indicated on the accompanying maps. Again, all of the belts and areas of 
acid eruptive rocks, such as the central area of the Porcupine Mountains, 
and the great spread of red rock in the Brulé Lake country in Minnesota, 
are without copper. The same is true also of all belts and areas of coarse- 
grained basic rocks, such as the great area of coarse gabbro in the Bad 
River country in Wisconsin and the similar area which occupies so large a 
belt of country between Duluth and Brulé Lake in Minnesota. The favor- 
able phase of the formation for the existence of copper in any form of deposit 
is the thin and regularly bedded one, with well-developed amygdaloids. 
