The Geology and Petrography of Crater Lake National Park, 1902
LAVAS OF MOUNT MAZAMA.a
Only two of the eleven dikes of Mount Mazama are dacite. They cut the older andesites (57) and connect directly with the thickest part of the Llao Rock flow. One of the dikes has a thickness of 10 feet. In the middle (130) it is holocrystalline and contains crystals of hornblende, but upon the border it is black and glassy, a vitrophyre (131), like most of Llao Rock. The other dike is regular, varying from 2-1/2 to 4 feet, and the material closely resembles the last in all respects. One of these, or possibly both, contributed to the escape of the great mass of Llao Rock.