phenocryst

Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 11
by James St. John

Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 4
by James St. John

Black porphyritic metarhyolite (Utley Rhyolite, Paleoproterozoic, 1765 Ma; Utley Rhyolite Quarry, near Green Lake, Wisconsin, USA) 3
by James St. John

Volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA) 2
by James St. John

Quenched blobs in pinkish-reddish porphyritic dacite (Upper Pleistocene, 27 ka; Devastated Area, Lassen Volcano National Park, California, USA) 5
by James St. John

Volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA) 9
by James St. John

Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 43
by James St. John

Quenched blobs in pinkish-reddish porphyritic dacite (Upper Pleistocene, 27 ka; Devastated Area, Lassen Volcano National Park, California, USA) 10
by James St. John

Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 10
by James St. John

Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 29
by James St. John

Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 30
by James St. John

Pilot Butte (Pliocene madupite lamproite volcanic center in the Leucite Hills, Wyoming, USA) 5
by James St. John

Porphyritic rhyodacite (Holocene; Chaos Jumbles Landslide, Lassen Volcano National Park, California, USA) 6
by James St. John

Volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA) 18
by James St. John

Partially devitrified rhyolitic obsidian (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 14
by James St. John

Vesicular andesite (Upper Pliocene, 1.9 Ma; Sierra Grande Shield Volcano, Raton-Clayton Volcanic Field, New Mexico, USA) 10
by James St. John

Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 23
by James St. John

Hot Creek Rhyolite Flow (Pleistocene, 288 or 333 ka; Hot Creek Gorge, Long Valley Caldera, California, USA) 18
by James St. John

Quenched blobs in pinkish-reddish porphyritic dacite (Upper Pleistocene, 27 ka; Devastated Area, Lassen Volcano National Park, California, USA) 11
by James St. John

Madupite lamproite (Late Pliocene, 3 Ma; Pilot Butte volcanic center, Leucite Hills, Wyoming, USA) 1
by James St. John

Porphyritic mica peridotite (Paleozoic; Tompkins County, New York State, USA) 1
by James St. John

Obsidian & devitrified obsidian gravel (Obsidian Cliff, Yellowstone, Wyoming, USA) 8
by James St. John

Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 21
by James St. John

Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 26
by James St. John

Porphyritic basalt (Middle Pleistocene, 670 ka; Black Rock Volcano, Millard County, Utah, USA) 3
by James St. John

Kimberlite (Gates-Adah Kimberlite Dike, Early Jurassic; Fayette County, Pennsylvania, USA) 10
by James St. John

Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 24
by James St. John

Rhyolitic obsidian (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 1
by James St. John

Volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA) 20
by James St. John

Kimberlite (Gates-Adah Kimberlite Dike, Early Jurassic; Fayette County, Pennsylvania, USA) 17
by James St. John

Porphyritic shoshonite (Denver Formation, 63-64 Ma, Lower Paleocene; Table Mountain, Golden, Colorado, USA) 3
by James St. John

Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 24
by James St. John

Crater Lake Caldera & Mount Scott Volcano (Oregon, USA) 4
by James St. John

Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 26
by James St. John

Vitrophyre (Braddock Peak Volcanic Complex, Upper Oligocene, 27.7 Ma; Lava Cliffs, Rocky Mountains National Park, Colorado, USA) 2
by James St. John

Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 7
by James St. John

Dome F of Chaos Crags volcanic domes (Holocene; Lassen Volcano National Park, California, USA) 1
by James St. John

Kimberlite (Gates-Adah Kimberlite Dike, Early Jurassic; Fayette County, Pennsylvania, USA) 12
by James St. John

Kimberlite (Gates-Adah Kimberlite Dike, Early Jurassic; Fayette County, Pennsylvania, USA) 11
by James St. John

Rhyodacite vitrophyre (Llao Rock Rhyodacite, Holocene, ~7.8-7.9 ka; roadcut near Llao Rock, margin of Crater Lake Caldera, Oregon, USA) 51
by James St. John

Llao Rock (Crater Lake Caldera, Oregon, USA) 31
by James St. John

Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 22
by James St. John

Vesicular andesite (Upper Pliocene, 1.9 Ma; Sierra Grande Shield Volcano, Raton-Clayton Volcanic Field, New Mexico, USA) 11
by James St. John

Porphyritic rhyolite (Cenozoic; Chaffee County, Colorado, USA) 3
by James St. John

Wizard Island Cinder Cone (Holocene, ~7.2 ka; Crater Lake Caldera, Oregon, USA) 20
by James St. John

Vitrophyre (Braddock Peak Volcanic Complex, Upper Oligocene, 27.7 Ma; Lava Cliffs, Rocky Mountains National Park, Colorado, USA) 1
by James St. John

Columnar-jointed rhyolitic obsidian lava flow (Roaring Mountain Member, Plateau Rhyolite, Upper Pleistocene, ~59 ka; Obsidian Cliff, Yellowstone, Wyoming, USA) 31
by James St. John

Basalt (McCartys Flow, Holocene, ~2.4 to 3.9 ka; southeast of Grants, New Mexico, USA) 6
by James St. John

Lugarite (Lugar Sill, Early Permian, 288 Ma; near Lugar, Ayrshire, Scotland) 14
by James St. John

Llao Rock (Crater Lake Caldera, Oregon, USA) 10
by James St. John

Vesicular andesite (Upper Pliocene, 1.9 Ma; Sierra Grande Shield Volcano, Raton-Clayton Volcanic Field, New Mexico, USA) 2
by James St. John

Hot Creek Rhyolite Flow (Pleistocene, 288 or 333 ka; Hot Creek Gorge, Long Valley Caldera, California, USA) 20
by James St. John

Rhyodacite vitrophyre (Llao Rock Rhyodacite, Holocene, ~7.8-7.9 ka; roadcut near Llao Rock, margin of Crater Lake Caldera, Oregon, USA) 1
by James St. John

Metanorthosite (Mesoproterozoic, ~1.15 Ga; Cranberry Lake, New York State, USA) 2
by James St. John

Porphyritic basaltic andesite (juvenile lava from a 14 June 2006 pyroclastic flow from Mt. Merapi, ~1200 m elevation at Bebeng, Java, Indonesia) 2
by James St. John

Black porphyritic dacite (upper Holocene, 14 May 1915; Devastated Area, Lassen Volcano National Park, California, USA) 15
by James St. John

Volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA) 12
by James St. John

Llao Rock (Crater Lake Caldera, Oregon, USA) 30
by James St. John

Volcanic breccia (Upper Oligocene, 25 Ma; Narbona Pass Maar Volcano, near the summit of the Chuska Mountains, Navajo Volcanic Field, New Mexico, USA) 8
by James St. John

Monchiquite lamprophyre dike (Precambrian; Michipicoten River Bridge South roadcut, south of Wawa, Ontario, Canada) 46
by James St. John