The Geology and Petrography of Crater Lake National Park, 1902
MIOCENE HISTORY OF CASCADE RANGE.
There can be no doubt, however, that during the Miocenea the volcanoes of the Cascade Range were most active and the greater portion of the range was built up, although it is equally certain that volcanic activity continued in the same region at a number of points almost to the present time. While it may be surmised that the volcanoes of the Cascade Range are extinct, there are many solfataras, hot springs, and fumaroles, showing that volcanic energy is not yet wholly dissipated. All the peaks of the Cascade Range were once active volcanoes, and from them came most of the lava of the range. Each great volcano was surrounded within its province, at least during the later stages, by numerous smaller vents from which issued the lava that filled up the intervening spaces and built up the platform of the range.
aTwentieth Ann. Rept. Rept. U.S. Geol. Survey, Part III, 1900, p. 32.
All of the great volcanoes of the range probably had their beginning in the Miocene. Many of them, like Lassen Peak and Mount Shasta, continued their activity into the Glacial epoch, and have suffered much erosion since their last eruptions. In this manner important structural differences have been brought to light among the peaks about the headwaters of the Umpqua, Rogue, and Klamath rivers, and these may be noted as throwing some light upon the history of Mount Mazama.