== 
ROCKS BETWEEN PORTAGE LAKE AND GRATIOT RIVER. 195 
Thickness in feet. 
Diabases and amygdaloids; including, at 670 feet above conglomerate 8, the 
Grand Portage cupriferous amygdaloid, and at 510 feet the Isle Royale 
cupriferous amygdaloid, largely worked on the south shore of Portage 
WANG. oo 2ag33.55, 1a So A OS Boe ORE OP OREO RO AOE e acer eae 2,050 
Conglomerate 8....... = ELD ic Sts) mae Mey aad Ss BPs alee ee en Sa | 2 
Dia AsessaAnNawamMy PAROS! ac. -2 ce .c css Ns eknc dais ciocmprewecpewewieweenee 420 
On PI OMEALCR EM sia aeis ys yoke cA era hel~ op ios ee nisl adjul siete otois aranis ease Sieeiersice 2 
Diabases and amygdaloids. .... Me rapsi tis uise[avapeinievelsistohe  siortorsterayuisiceters si stemiole eas 260 
WoOnP LOMET ALGO Meer felsrcict tte Fie = avian ansehen, aie 5, aiciea let See Bele e Jp ayecia ct 3 
MIaAhACesianGeAM VY OORLOIOS:: i= set (sss brewers viv c ab eteeierewieie Aeemess cee aoe ces 181 
Wonelomeratep rere eric jie = sees siecle w cise aisle’, © t-ieie sicieine: sslemivn cities emieie 24. 
Diabases and amygdaloids. .-.......-......- Be ayatale tee ese ieee SSS Doren 240 
Conglomerate rlamemetr sje sae he oes eee wanesis reds meeceticis cteteasect st 12 
Diabases and amygdaloids. ........-..... boi otoevate clare Mate on cinenetta es eisai aes 1,149 
@onglomeratereemer ects oss S es oc cece ac) cio scya slic sims esieiss eee Ae, eens > 56 
Diabases and amygdaloids............-. Sete ve cinibs ePana since tio mae slaaveente sje es 370 
@onglomerater2 easene sstecs feel ot Reece ncteecsctietsoces Wale facetemes: 35 
Wiahasesvandgamy Pa aloidsrea eS hss cele rials coe arses tists orsiesisieiino sist- see's 1,140 
Oonglomerateylitass sac ces 52t-se<essscae sciciciare sects spank qocomeonesecece 97 
PAmyodaloidesmeeemeesecisisse.s s<:6/s LER Ae aya s Sade vaste asccese beiene het 14 
ADO ballipaaerresnewsce seis ei sicias: slasnvetelo’ iste clew wie sie sats Sela ie einisis, cislaje oiels Sainte 22, 680 
The comparatively rare exposures to the northeast of Portage Lake 
and to the southwest of Gratiot River exhibit only a few points of particu- 
lar interest. One of the most striking of these is the great change, already 
alluded to, that takes place in the nature of the conglomerate about six miles 
from Portage Lake. About Portage Lake the greatly predominant pebbles 
are compact felsites or porphyries with only feldspars, and no visible quartz, 
while augite-syenites are found in some beds. Porphyries with visible 
free quartz are unknown. Towards Calumet, however, all of the conglom- 
erates show true quartz-porphyry predominating among the pebbles, while 
still further to the northeast the quartzless porphyries again predominate, 
although not to the exclusion of those with visible quartzes. This change, 
as pointed out by Pumpelly’ and Marvine,’? must be due to a variation in 
the nature of the source, in a direction parallel to the general trend of 
Keweenaw Point. This source Marvine and others would place north of 
the point where now lie the waters of Lake Superior. I have already 
shown several times that the porphyry conglomerates derived their pebbles 
1Geol. Survey of Mich., Vol. I, part II, p. 17. ?Geol. Survey of Mich., Vol. I, part I, p. 60. 
