FOR convenience in description, the dacites of the Crater Lake region may be divided into three series. The youngest includes the vast deposits of dacite pumice erupted just before the formation of the caldera. An older series includes deposits of pumice and viscous domes and flows erupted from a semicircular arc of vents on the northern flank of the volcano; to this series belong the lavas of Llao Rock, Cleetwood Cove, Grouse Hill, and Redcloud Cliff. The third and oldest group of dacites includes the dacite pumice interbedded with andesites on the caldera walls, together with the extensive sheets of dacite lava and dacite domes on the south and east slopes of the volcano. In this section, only the third group is to be discussed.

Before any of these eruptions of dacite began, Mount Mazama was already a giant andesitic cone, and had almost attained its maximum height. Mount Shasta had likewise reached its greatest elevation before dacites commenced to escape from fissures far down its sides. Moreover, the only dacitic material erupted from the summit of Mount Mazama was in the form of pumice; the dacite flows and domes all issued from subterminal fissures.