Figure F103. Photomicrograph, under plane-polarized light, of siltstone from just beneath the Subunit 5B/5C boundary. This unusual cross-laminated siltstone is probably an altered tuff. A relict grain that exhibits vesicular texture is circled. In cross-polarized light, this grain shows low birefringence, suggesting that it was glass that was altered to clay minerals. Surrounding white areas are authigenic carbonate, whereas darker material is low birefringent to isotropic; both are considered to be alteration products (Sample 210-1276A-75R-6, 37-40 cm).

Figure F103. Photomicrograph, under plane-polarized light, of siltstone from just beneath the Subunit 5B/5C boundary. This unusual cross-laminated siltstone is probably an altered tuff. A relict grain that exhibits vesicular texture is circled. In cross-polarized light, this grain shows low birefringence, suggesting that it was glass that was altered to clay minerals. Surrounding white areas are authigenic carbonate, whereas darker material is low birefringent to isotropic; both are considered to be alteration products (Sample 210-1276A-75R-6, 37-40 cm).