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Porphyritic andesite of probable Eocene age, northern Crazy Mountains, Montana. Blog posts relevant to this rock: www.nvcc.edu/home/cbentley/geoblog/2009/08/those-mountains-are-crazy.html and blogs.agu.org/mountainbeltway/2011/03/09/a-portrait-of-the-feldspar-as-a-young-mineral /
This porphyritic (olivine phenocrysts) vesicular basalt was collected from a roadcut on the southwest rift zone of Mauna Loa in Hawaii. I believe that it is from the 1907 lava flows in that area, though I'm not certain of that. I'm curious to see how the focus will work on a relatively flat (but not cut) surface. Th...