PORTAGE LAKE SECTION. 189 
higher. Certain portions of the thickness are marked by certain distinguish- 
ing lithological characteristics, 
which, without in any instance being peculiar to a given horizon, still serve to 
mark decidedly those parts of the series where they are, respectively, most frequent- 
Thus, to begin towards the eastern part of the field, from the neighborhood of “ Mabb’s 
vein”’ to within, say, 1,000 feet east of the Isle Royale “ vein,”! there is a tendency, 
among the different traps, to a compact or fine-grained texture with a dark-green, 
almost black color, sometimes slightly mottled, especially on the weathered surface. 
* * * From this region till 1,500 feet or more west of the Isle Royale copper-bearing 
bed, the upper portions of very many of the beds have the amygdaloidal cavities filled 
with a light-greenish white or pale pink prehnite, which sometimes, for a width of 2 to 6 
feet, form 10 to 40 per cent. of the rock, and lend it a very characteristic spotted 
appearance. During the next 2,000 feet or more, the traps have frequent seams 3 to 
20 inches thick, consisting of distinctly individualized triclinic feldspar, delessite, preh- 
nite and specular iron; these occur both parallel to the plane of the bedding and 
oblique to it. The traps through a portion of this distance are frequently impregnated 
with epidote, as is also the cement of the conglomerate beds. On the ‘“‘ Dacotah” prop- 
erty we come to a belt of the formation in which many beds have a tendency to a 
coarse-grained, crystalline texture, and in some, the character is highly developed. 
* * * * Still further west, on the “South Side” property, the brown amygdaloids 
often present a scoriaceous appearance which is quite characteristic. Some, at least, 
of these features are traceable for miles in the longitudinal extension of the zones in 
which they occur.’ 
Below the upper limit of Pumpelly’s Portage Lake section—conglomer- 
ate 22—he recognizes twenty interstratified porphyry conglomerates, which 
he numbers from below upwards; the twenty-first, or No. 13, not havy- 
ing been met with on Portage Lake. It is the well-known Calumet con- 
glomerate. Of the Portage Lake conglomerates, three—Nos. 1, 2, and 
8—about 50 feet thick each, lie in the lower 2,900 feet. ‘Then follows a 
zone of about 1,300 feet, in which there are five conglomerates—Nos. 4, 
5, 6, 7, and 8—from 2 to 20 feet thick. Next follow 2,250 feet without con- 
glomerates. Then for 5,000 feet in thickness are eight conglomerates from 
200 to 1,000 feet apart, and from mere seams to 30 feetin thickness. ‘These 
are Nos. 9, 10, 11, 13, 14, 15, 16, and 17. Then four conglomerates—Nos. 18, 
19, 20, and 21—ageregating 200 feet in thickness, in a zone of only 550 feet. 
In the total thickness of 12,000 feet measured, the conglomerates aggregate 
about 580 feet, in twenty-one different layers.’ According to Pumpelly, 
'Copper-bearing amygdaloids. 
2 Geological Survey of Michigan, Vol. I, Part II, pp. 17, 18. 
3 The figures here given are taken from Pumpelly’s sections in the Atlas of the Geological Survey 
of Michigan, and do not in all respects conform with those given by Marvine in Vol. II, Geol. Surv. 
Mich., p. 61. 
