288 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
taken by some as still more conclusive proof of eruptive origin—they are 
identical, even to the same crowding of the olivine in the interspaces, with 
the rocks of the narrow dikes found along the coast westward to Duluth. 
As shown below, the rock of the dikes observed cutting the Agate Bay beds 
themselves is a true luster-mottled melaphyr. 
The subordinate layers, of which both amygdaloids and compact por- 
tions appear to be made up, are never regular or persistent, and in this 
respect there is a contrast with the subordinate layers of true sedimentary 
rocks, the most irregular of which are never so irregular as these. When 
in the field it was thought that the peculiar combination of resemblances to 
sedimentary beds and to the usual eruptive rocks of the region shown in 
these stratiform amygdaloids and associated massive layers, might find ex- 
planation in the origin of the amygdaloids as volcanic ashes,’ 2. ¢., frag- 
mental volcanic material stratified by water. In this case the amygdules 
might be pseud-amygdules, which could of course originate as well from 
fragmental basic material as from the same material in the original massive 
condition. But the study of specimens, and more especially the micro- 
scopic study of thin sections, which develops the wholly non-fragmental 
character of the material, and the true vesicular nature of the amygdaloids, 
show that such an idea is wholly untenable. 
The following section, made out along the cliffs just west of Agate Bay, 
serves well to show the sort of succession everywhere to be observed in these 
stratiform beds The section could have easily been extended both up and 
down, but is sufficient as an illustration. The layers dip some 6° to 8° to 
the southeast. The order is an ascending one. 
IA. Massive, vertically columnar layer of a medium-grained, distinctly at 
crystalline, purplish rock, mottled dark and light on a weathered 
surface. Sparsely scattered pseud-amygdules of calcite and 
laumontite, and of chlorite, are contained. The thin section shows 
plagioclase; augite, inclosing plagioclases; magnetite; olivine, 
wholly altered to red oxide of iron and a green substance, and 
crowded with the magnetite between the augites; and pseud- 
amygdules of a pale greenish substance. Grading into the over- 
lying rocks by increase of abundance of amygdules. Thickness 
seen Sbove water is. ctt sete e a ee cee eee eee ees tater 5 
‘Norwood seems to have had this idea with regard to some of these beds, though most of them 
he regarded as ‘“‘metamorphic shales.” See Owen’s Geological Survey of Wisconsin, Iowa, and Min- 
Desota, p. Jol. 
