The Mirror Lake Intrusive Breccia cuts Upper Triassic rocks of the Seven Devils Volcanics. Plutonic clasts, mostly of “mafic” and “granitic” compositions, are suspended in a hornblende metadiabase porphyry matrix. “Mafic” clasts are hornblende metagabbro and metadiorite, and “granitic” clasts are metamorphosed quartz diorite and albite granite. The plutonic clasts are representative of underlying plutons which had been intruded in Late Triassic–Middle Jurassic time. The intrusive breccia also was injected during that time interval. Devolatilization of a water-rich magma gave impetus to the rising mass and pieces of wall rock were broken off and carried upward. Clasts were rounded by corrasion and assimilation during transport and magma crystallization. Later regional metamorphism produced greenschist mineralogy in the rocks.