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From their shape such casts are called amygdules (Greek, amygdalon, an almond).
The flows carry native copper and copper sulphides in minutely disseminated form and in amygdules, but apparently not in quantities sufficiently concentrated to mine.
The cellular and slaggy surface where the pasty lava was distended by escaping steam is recognized by the amygdules which now fill the ancient steam blebs.
Most AGATES are banded amygdules in which the silica has been laid in varicolored, concentric layers.
After the Keweenawan was uplifted to form land, percolating waters leached out much of the copper diffused in the lava sheets and deposited it within steam blebs as amygdules of native copper, in cracks and fissures, and especially as a cement, or matrix, in the interbedded gravels which formed the chief aquifers of the region.