NOTES. 433 
the occurrence of basic flows superposed upon intermediate flows; and (9) the inter- 
section of acid by basic rocks. 
(1.) Lhe position of the different kinds of eruptives in the general stratigraphy of the 
series.—Acid rocks, while on the whole decidedly affecting low horizons in the series, 
here and there occur at quite high ones, as, for instance, in the Poreupine Mountain 
region (pp. 155, 206—224, and Plates XIX, XX, and XX1J), and on Michipicoten Island 
(pp. 155, 342, 343). In the latter case the acid rocks occur at the summit of a series 
of over 18,000 feet of plainly bedded eruptive flows, both intermediate and basic, 
with interstratified conglomerates and sandstones. But, wherever occurring, at low 
horizons or high, the beds of acid rocks have commonly above and below them basic 
beds. The instances of such an occurrence of acid rocks are altogether too numer- 
ous for all to be here referred to. I may merely mention the following cases: Mount 
Houghton, on Keweenaw Point (pp. 181-183, Plates XVII, XVIII), is a mass of red 
felsite, having both on the south, or below it, and on the north, or above it, great 
thicknesses of basic flows. The belt of red porphyry which forms so prominent a 
feature to the west of the Ontonagon River, and as far as the Bad River in Wisconsin, 
has in a similar manner basic flows both above and below it (pp. 199, 209, 220, 231, 
Plates XXII, XXIII. See also Geol. of Wis., III, pp. 195, 198, Plate XVI, and Atlas, 
Plate XXII). On the Minnesota coast the larger number of flows of acid rocks occur 
in the subordinate series of beds which I have called the Beaver Bay Group (pp. 298- 
323). Both above and below the Beaver Bay Group occur great thicknesses of basic 
flows (Agate Bay Group, Temperance River Group, pp. 267, 268, and Plate XX VI); 
besides which, within the Beaver Bay beds themselves, the acid rocks are found over- 
Jain and underlain by basic flows. 
But not only is it plain from their stratigraphical relations that the acid and basic 
eruptives alternated with one another in formation; the flows of intermediate acidity 
also evidently alternated, as to time of formation, with both basic and acid flows, for 
the beds of intermediate acidity occur at many horizons throughout the series. For 
instance, the peculiar, resinous-looking diabase-porphyrites of the south side of Michi- 
picoten Island (pp. 86, 87, 343), having about 60 per cent. of silica, lie above many 
thousand feet of basic flows. Numerous instances of the occurrence of sub-basic 
diabase-porphyrites on the Minnesota coast might also be cited in this connection, as, 
for instance, the brown diabase-porphyrite, with strongly-developed amygdaloid, which 
forms the shore cliff one mile below the mouth of Silver Creek, NE. 4 Sec. 22, T.53, 
R. 10 W., Minnesota (pp. 80, 54, 285). An interesting case of the interstratification of 
a rock of intermediate acidity with basic kinds is furnished by the trap belt which 
overlies the felsitie porphyry of the Porcupine section (pp. 209, 214, 217, Plates XIX, 
XX, XXI). This belt has a surface width of one-fourth to one-third miles, and a thick- 
ness of from 300 to 500 feet. Towards the middle of the thickness a porphyry-con- 
glomerate is included, with a thickness of over 60 feet. The rocks of this belt include 
diabases and diabase-amygdaloids of the ordinary types, and luster-mottled melaphyrs. 
Interstratified with these, both above and below the intermediate conglomerate, are 
layers of a diabase-porphyrite which is distinctly of intermediate acidity, containing, 
so far as determined, about 60 per cent. of silica. Clearly, then, we have here an acid 
porphyry succeeded by flows of a wholly basic material, following which come, in as- 
cending order, arock of intermediate acidity, others which are completely basic, a por- 
