434 COPPER BEARING ROCKS OF LAKE SUPERIOR. 
phyry-conglomerate, basic flows, again more rocks of intermediate acidity, more basic 
rocks, and finally a great thickness of porphyry-conglomerate and sandstone. No 
possible explanation of this section can be offered by which a succession of intermedi- 
ate, acid, and basic eruptions can be made out, for even if the acid porphyry lying at 
the base of the section should be taken as intrusive, and therefore possibly subsequent 
to the overlying rocks, there remains the intervening porphyry-conglomerate to prove 
the existence of acid porphyries prior to the eruption of both the intermediate and 
basic rocks of this belt; whilst the diabase-porphyrite cannot in any way be made out 
as antecedent to all of the basic rocks with which it is immediately associated. 
It should be said that in all cases here cited of the occurrence of rocks of inter- 
mediate acidity, care has been taken to refer only to those in which the intermediate 
acidity is plainly an original character, and not in any measure one due to a subse- 
quent infiltration of secondary quartz. 
The question might arise in some minds as to whether the cases here cited of the 
indiscriminate stratification of acid, intermediate, and basic rocks might not be due to 
the subsequent intrusion in the form of sheets of all of the basic rocks concerned. 
That some of the basic rock beds of the series, and especially those formed of coarse- 
grained rocks, may be of an intrusive nature, has been indicated in the memoir (pp. 27, 
144), though definite evidence of this is lacking. However this may be, in the present 
connection, care:has been taken to consider only those basic rocks which are furnished 
with well-developed amygdaloids and are consequently the results of flowage at 
the then existing surface in each case. Indeed, should we, for the sake of argument, 
admit—what Ido not at all believe—that all of the basic and intermediate beds which 
are not furnished with amygdaloids are intrusive, those furnished with amygdaloids 
being always taken as surface flows, we should immediately find ourselves at the saine 
result, namely, that there has been no definite order among the eruptions of different 
acidity. 
Many more instances than are here mentioned might be cited, but it is thought 
that those given are sufficient for the sake of the argument. 
The Huroniap, beneath the Keweenawan, contains many beds of eruptive mate- 
rial. True acid rocks are extremely rare, if indeed they occur at all, but basic and 
intermediate eruptives are plenty. There is much doubt, with our present knowledge 
of them, as to how far these eruptives are intrusive, and though it is not deemed prob- 
able, some of them may be intrusive sheets, contemporaneous with the surface flows 
of the Keweenawan. Many of these beds partake of the folds of the folded Huro- 
nian and hence antedate the folding. On the whole, it now seems probable that by far 
the greater part of the Huronian eruptives preceded all of the Keweenawan eruptives, 
acid and basic. 
(2.) The occurrence of acid flows directly and visibly superposed upon basic flows.— 
Acid rocks directly overlying basic flows are met with m several places on the Min- 
nesota coast, but in the case of the Great Palisades, fully described in the memoir (pp. 
146-148, 314-318, Figs. 23 and 24), the occurrence is so striking and conclusive that no 
others need be cited. To the descriptions given in the memoir, I may merely add here 
that analytical determinations made since these descriptions were in type show that 
the diabases {or rather diabase-porphyrites, since they contain much non-polarizing 
matter) underlying the quartz-porphyry of the Palisades belong with the more basic of 
