The Geology and Petrography of Crater Lake National Park, 1902
Mount Thielsen (elevation 9,250 feet), the Matterhorn of the Cascade Range, is 12 miles north of Crater Lake and rises about 2,000 feet above the general level of the crest of the range. It is built up of bright-red, yellow, and brown layers of tuff, interbedded with thin sheets of lava, and is cut by a most interesting network of dikes radiating from the center of the old volcano. No trace of a volcanic neck is present—the peak is but a remnant carved out of the lava and tuff cone surrounding the vent. After the final eruption the molten material withdrew from the cone before consolidation, so as to leave no volcanic neck corresponding to that of Union Peak. The subsidence within the chimney of Mount Thielsen after eruption must have been over 1,000 feet, for the sheets of lava effused from that vent reach more than 1,000 feet above the exposed throat of the old volcano.