Figure F6. A. In this close-up photograph, the perlitic texture of aphyric dacite (Unit 2) indicates that the groundmass consisted of volcanic glass originally. Alteration proceeded preferentially along and outward from the perlitic cracks generating a pseudoclastic texture in some domains of the sample. In hand specimen, this unit has a sugary appearance. The arcuate perlitic cracks are enhanced by alteration generating light gray "islands" in a dark gray, irregular network of clay, opaline silica, and minor pyrite forming an apparent matrix (interval 193-1188A-5R-1, 36-48 cm). B. In thin section, well-preserved perlitic texture with scattered microlites is present within the "islands," which are surrounded by alteration products. Width of view = 2.75 mm (Photomicrograph ID# 1188A

Figure F6. A. In this close-up photograph, the perlitic texture of aphyric dacite (Unit 2) indicates that the groundmass consisted of volcanic glass originally. Alteration proceeded preferentially along and outward from the perlitic cracks generating a pseudoclastic texture in some domains of the sample. In hand specimen, this unit has a sugary appearance. The arcuate perlitic cracks are enhanced by alteration generating light gray "islands" in a dark gray, irregular network of clay, opaline silica, and minor pyrite forming an apparent matrix (interval 193-1188A-5R-1, 36-48 cm). B. In thin section, well-preserved perlitic texture with scattered microlites is present within the "islands," which are surrounded by alteration products. Width of view = 2.75 mm (Photomicrograph ID# 1188A_13; thin section 3).