Basaltic lava flows are commonly vesicular near their tops and bases as the lava there chills quickly enough to trap gas bubbles. Over time, as these basalts are exposed to alteration by hydrothermal fluids, new minerals may be deposited in these void spaces to form amygdules. This sample of amygdaloidal basalt is from the 1.1 Ga Keweenawan Rift series currently outcropping along the east shore of Lake Superior and exposed beautifully in roadcuts along the Trans Canada Highway in the Batchewana Bay region of Ontario. The rounded amygdules of this particular piece of basalt are filled primarily with chlorite and calcite; elsewhere in this unit epidote is a common amygdular mineral. Since these minerals are not commonly primary in basalts there is little chance of mistaking these amygdules for phenocrysts, which can also be seen in this rock (the small white laths of plagioclase).
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