Figure 8.--Photomicrographs of sandstones in the Minturn and Sangre de Cristo Formations: A) potassic arkosic wacke (21 pct potassium feldspar, 10 pct plagioclase, 19 pct matrix) with angular to subround quartz grains, sample 31G, medium sand, 1181 m below top, Minturn Formation, Marble Mountain section; B) sodic arkosic wacke (9 pct potassium feldspar, 27 pct plagioclase, 12 pct matrix) with subround and round quartz grains, sample 8G, coarse sand, 826 m below top, Minturn Formation, Marble Mountain section; C) sodic arkosic metawacke (18 pct potassium feldspar, 28 pct plagioclase, 18 pct matrix) with metamorphic epidote and angular to subround quartz grains, sample 12M, coarse sand, 264 m above base, Sangre de Cristo Formation, Thirsty Peak section; and D) potassic arkosic wacke (32 pct potassium feldspar, 15 pct plagioclase, 28 pct matrix), sample 21B, diamictite composed mostly of angular sand in hematitic matrix, 583 m above base, Sangre de Cristo Formation, Eureka Mountain section. All photos taken under plain polarized light, potassium feldspar stained yellow by cobaltinitrite. Q, monocrystalline quartz; CQ, compound monocrystalline quartz; RQ, rounded quartz in (B); K, potassium feldspar; P, plagioclase, M, mica (biotite marked, but muscovite also present); and R, plutonic rock fragment in (D).