Specimen Mountain, an eroded Cenozoic volcano in the northwestern corner of Rocky Mountain National Park, Colorado, is flanked on the south and west by an extensive series of faulted and folded lava flows and pyroclastics. Regional studies indicate that the erupted materials spread laterally for several miles in all directions from a single central conduit and buried a prevolcanic surface of considerable relief.

The volcanic rocks, as shown by analyses, are quartz latite and silicic rhyolite. Quartz latite flows are chilled samples of a magma which by gravitative differentiation gave rise to silicic rhyolite occurring as flows and plug rocks. Thick beds of ash, agglomerate, and breccia attest to intermittent explosions of great magnitude.

Inconclusive evidence suggests a Miocene or Pliocene age for the volcanics.

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