Periodic violent eruptions from many different centers during Cenozoic time deposited vast quantities of pyroclastic material as ash-flow tuffs over most of Oregon, although the Coast Ranges and isolated patches elsewhere in the state appear to have been spared these recurring inundations. Eruptions occurred at different times throughout the Cenozoic, and for purposes of description, they can be separated into three age groups: an older one of Eocene, Oligocene, and Miocene age, an intermediate one of early and middle Pliocene age, and a young group of late Pliocene, Pleistocene, and Holocene age. Some of these ash-flow tuffs are of small volume, less than a cubic mile, and are related to fissure vents, small domal complexes, or calderas from which several kinds of volcanic products were erupted. A few cover thousands of square miles, have volumes of tens of cubic miles, and apparently are related to a large-scale basinal collapse structure and associated calderas. Most ash-flow tuffs are rhyolite or dacite; a few are peralkaline soda rhyolite. Older ash-flow tuffs are commonly diagenetically altered to a variety of secondary minerals; of the younger tuffs, only those that erupted into shallow lakes exhibit comparable alteration.