Serpentine occurs as lenses from 50 feet to several miles across in the pre-Cambrian rocks forming the cores of the anticlinal mountain ranges of central and northwestern Wyoming. The serpentine is younger than metamorphosed sediments, is cut by metadiabase dikes, and both are cut by granite and pegmatites.Chrysotile occurs in the serpentine both as cross fiber and slip fiber in veins elongated with the lens. Cross fiber up to 1 1/2 inches long was found; most of it is short. A small tonnage has been produced from three deposits.Microscopic examination of brittle chrysotile shows extensive replacement by quartz. Harsh chrysotile shows fine-grained quartz in rods or veinlets parallel to fiber length and in cross vein-lets. Nearby pegmatites could have furnished the silica. The opinion is expressed that brittleness and harshness in chrysotile from the serpentine type of deposit is the result of introduction of silica with or without chemical reaction to form talc.The chromite of Casper Mountain occurs disseminated, and in bands or lenses in tremolite-chlorite-talc schist, originally an ultrabasic rock. The best ore would require concentration to meet market requirements. Some laboratory concentrates contain sufficient Cr 2 O 3 for the manufacture of ferrochrome; others contain an excessive amount of iron.