436 COPPER BEARING ROCKS OF LAKE SUPERIOR. 
(9.) The intersection of acid by basic rocks.—Dikes of basic rocks are to be observed 
cutting acid porphyries at a number of places on the north shore, as for instance near 
Lester River (p. 283), at Beaver Bay (p. 307), at Grand Marais (p. 320), near Red Rock 
Bay (p. 322). In the last case the dike-rock is a diabase-porphyrite with only 45.8 
per cent. of silica, whilst the rock intersected is a typically developed quartz-porphyry. 
NOTE 5. 
(Page 32.) 
COMPARISON BETWEEN TERTIARY AND KEWEENAWAN ERUPTIVES. 
In a first brief announcement of the results of my study of the Keweenaw Series 
(Report of the Director of the United States Geological Survey for the year ending 
June 30, 1881, p. xxxiii), Director Powell says that the acid eruptives of the Kewee- 
nawan “are regarded by Irving as ancient rhyolites and trachytes, from the degrada- 
tion of which the conglomerates of the series have resulted.” In the notes I furnished 
the Director as to my results, I had not meant to convey the meaning that I regarded 
the Keweenawan acid eruptives as lithologically identical or equivalent with the Ter- 
tiary rhyolites and trachytes—though his words perhaps might be so understood— 
but merely to indicate that they occupied the same general position as to acidity and 
general lithological charaeters among the Keweenawan eruptives as are occupied 
among the Tertiary eruptives by the rhyolites and trachytes. My acquaintance with 
the Tertiary eruptives is too limited to allow of my passing an opinion on the accuracy 
of the view commonly accepted by the most eminent lithologists of the day, viz, that 
the Tertiary eruptives are always distinct and deserving of separate names from those 
that preceded Tertiary time. 
NFO}, E16: 
(Page 32, eighth line from bottom; also page 138, last paragraph.) 
VOLCANIC ASH IN THE KEWEENAW SERIES. 
Mr. A. R. GC. Selwyn has published (Science, Vol. I, No, 1, February 9, 1883; also, 
Vol. I, No. 8, Mareh 30, 1883), since this volume was in type, a statement that volcanic 
ash exists in the Keweenawan rocks of Michipicoten Island, but he has not yet pub- 
lished any description of this material. Macfarlane, in his descriptions of the Geology 
of Michipicoten Island (Geol. Surv. Canada: Report of Progress, 186366, p. 138 et 
seq.), Speaks of “ breccias,” which, as I understand him, are the rocks to which Selwyn 
refers. Macfarlane describes the breccia in one case as consisting of ‘ small fragments 
of melaphyr, some fresh looking, but the greater part bleached to a reddish-gray color, 
inclosed in a reddish-brown earthy matrix, consisting most probably of finely commi- 
nuted melaphyric material, as it is readily fusible before the blow-pipe.” In another 
case he speaks of ‘a trap breccia, composed of fragments of dark-brown melaphyr, 
cemented together by a brownish-red trappean sand.” Ihave not observed anywhere 
on the south or north shores of the lake any rocks which resemble these, if I under- 
stand Macfarlane’s descriptions correctly, unless they are somewhat like the Nonesuch 
sandstone of the Porcupine Mountains. I have said in the text that this sandstone may 
be in part of volcanic ash nature. Though it is not impossible that the Michipicoten 
