Figure 9-3. Layer of microcrystalline CaCO3 (M) that includes a quartz grain (blue arrow at right). Note the abrupt lateral termination (red arrow) of the microcrystalline layer against adjoining columnar calcite (C). A corresponding image in cross-polarized light shows that the columnar calcite extends through the area to the left of the microcrystalline layer, and that another quartz grain is present at the left blue arrow, in the columnar calcite but at a position laterally equivalent to the microcrystalline layer.      Photomicrograph was taken in plane-polarized light; field of view is 2.3 mm wide. Carthage, Tennessee, U.S.A.; Stalagmite MR68; thin section MR68. Sample collected from highway construction debris by L. Bruce Railsback.
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