. Bulletin of the Geological Society of America. Geology. ORDOVICIAK SYSTEM 411 late on the slopes of the Deadwood formation. On Dinwoody creek the massive dolomite, 50 feet thick, lies on 100 feet of shaly limestone that is very sandy. Owl CreeJc mountains.—In the eastern part of the Owl Creek uplift the Bighorn limestone is ahont 40 feet thick; in Phlox moimtain and Owl Creek canyon it is over 150 feet, and near Crow creek it is alwut 100 feet thick. The outcrop is continuous around the liigher central area of the uplift, and it extends westward along South fork of Owl creek to a point 3 mil
. Bulletin of the Geological Society of America. Geology. ORDOVICIAK SYSTEM 411 late on the slopes of the Deadwood formation. On Dinwoody creek the massive dolomite, 50 feet thick, lies on 100 feet of shaly limestone that is very sandy. Owl CreeJc mountains.—In the eastern part of the Owl Creek uplift the Bighorn limestone is ahont 40 feet thick; in Phlox moimtain and Owl Creek canyon it is over 150 feet, and near Crow creek it is alwut 100 feet thick. The outcrop is continuous around the liigher central area of the uplift, and it extends westward along South fork of Owl creek to a point 3 miles west of longitude 109°. It shows again on the slopes ad- joining Crow Creek canjon and tlie upper portion of West fork of Muddy creek and in Bighorn canyon. Tlie most prominent exposures are in the great escarpment of Phlox mountain. It is a hard, massively hedded dolomite, mostly of a light huff color, hut somewhat darker when weath- ered, filled with a coarse network of irregular siliceous masses, mostly from a half to 1 inch in diameter. On Aveathering, this material stands out half an inch or more on the rock surface as a ragged network, the purer rock between having been dissolved. This feature and the very massive bedding are characteristic. In Owl Creek canyon the massive limestone is overlain by 20 feet of white limestone, capped by a 20-foot massive bed similar to the thick limestone below, while at the top of the formation there are a few feet of sandstone and shale. In places these upper beds weather to a reddish tint, strongly suggestive of the member of Eichmond age which occurs in the northern portion of the Bighorn uplift. The appearance of two of the most prominent outcrops in the Owl Creek canyon is shown in plates 76 and 77 of my memoir on Ordovician of the Northwest.® Fossils and age.—Fossils usually are rare in tlie Bighorn limestone, consisting mostty of fragments of maclurina and corals. At one locality in the Wind Eiver mountains, however, Mr Woo
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