Figure 15A | Table of Contents

Figure 14. Close-up photograph of a perlitic texture of aphyric dacite (Unit 2) indicates that the groundmass originally consisted of volcanic glass (interval 193-1188A-5R-1, 35–49 cm). Alteration proceeded preferentially along and outward from the perlitic cracks generating a pseudoclastic texture in some domains of the sample. A. 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, silica, and minor pyrite forming an apparent matrix. B. In thin section, well-preserved perlitic texture is present within the "islands," which, hence, represent remnants of the originally glassy groundmass (field of view = 2.75 mm).