Reconnaissance geology of the 500-square-mile wilderness north of the Skagit River, and the mineral survey that was made before it became a park. 1972
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/geology/usgs/publications/bulletin-1359/usgs.gov.md
Fine-grained thinly foliated greenschist and black phyllite occur in a north-northwest-trending belt along the west edge of the area. The rocks are particularly conspicuous near Bacon Peak and in the vicinity of Mount Shuksan.