The indigenous inhabitants of what is now called Northern Michigan used the metallic copper they found at or near the earth’s surface for at least 6,000 years to make tools and ornaments. There was no need for what we today call mining, with its implications of underground digging, explosives and other modern technological innovations.
Michigan’s native metallic copper comes in many shapes and sizes, from pebbles to large boulders. There was a large crystal of copper at the museum entrance that did not have a cubic form. I was really confused until I ran across the exhibit on pseudomorphs, which explained that a mineral can fill in a void in a rock that used to be occupied by a different mineral but has since dissolved away. Then its shape will replicate that of the original substance.
In addition to the museum, we visited several beaches along Lake Superior and Portage Lake. Figure 4 shows a large pebble that I found at a beach, which has a dark matrix with pink, white and green inclusions. These “amygdaloidal basalts” are volcanic rocks that contain gases when they start out as molten lava. They solidify as a foam. Over time, other minerals can fill in the holes. Basalt can sometimes oxidize, which can change the color to reds or browns. The smaller pebble shown in Figure 5 may be an oxidized amygdaloidal basalt.
1. Martin, Susan R., Wonderful Power, Wayne State University Press, 1999, p. 78