a6 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
columnar, fine-grained, chocolate-brown, semi-conchoidal diabase, with 
sparse laumontite and calcite amygdules increasing in number above, and 
AG Ios. 
ay 
ot Sl 
eel [7 
Th, a 
Phd Le 4 Co l 
= A ee WA My) 
pitied ALY 
Watley lire 
Fic. 24.—Section on south cliff of the Great Palisades, Minnesota coast. A, amygdaloid; B, columnar 
diabase-porphyrite; C, mingled amygdaloid and detrital matter; D, columnar diabase-por- 
phyrite; E, amygdaloid; F, quartz- porphyry. 
6 feet of true amygdaloid, of which the upper 3 feet is highly vesicular. 
C is a thin seam (6 inches) of mingled amygdaloid and detrital matter. D 
is another great flow of diabase, made up of 1 foot of basal amygdaloid or 
vesicular portion, 30 feet of massive cross-columnar diabase, as above, the 
sparse amygdules gradually increasing in number upwards, and 10 feet of 
true amygdaloid (E) growing more and more vesicular upwards. 
The compact diabase of these beds belongs with the diabase-porphy- 
rites, the augite being a quite subordinate ingredient, while there is often 
much non-polarizing matter. A specimen from the upper bed yielded 47.9 
per cent. of silica. In the upper or amygdaloidal portions of these beds 
are scattered small hard red patches, a few inches across. At times these 
patches are round, and appear at the first glance somewhat like the pebbles 
of a conglomerate, but they are more often irregular and are mingled curi- 
ously with the surrounding diabasic material. Some of the apparently 
rounded particles are plainly seen, even by the naked eye, to fill original 
vesicles, often of relatively large size, and in these cases are either on the 
outer wall of the vesicle with calcite within, or have between them and the 
wall a lining of calcite and laumontite. In the thin section this material is 
easily seen to be fragmental, being composed of subangular quartz grains, 
with some reddish interstitial matter. 
It is possibly this reddish matter that has caused Norwood and Win- 
chell’ to speak of the existence here of conglomerate and breccia rocks, 
1 Owen’s Geological Survey of Wisconsin, Iowa, and Minnesota. Philadelphia, 1852, pp. 259, 362- 
364. Seventh Annual Report of the Geological and Natural Hist. Survey of Minn., p. 10. 
