there are two principal facies of lower paleozoic rocks in nevada. in the eastern part of the state, a north-trending fossil-rich carbonate shelf of ordovician to devonian age, termed the carbonate or eastern assemblage, gives way westward to a contemporaneous expanse of siliceous sedimentary deposits and minor mafic volcanic rocks termed the siliceous or western assemblage. crafford assigned these two facies respectively to the shelf domain and the basin domain. the dark color of the western assemblage, the scarcity of carbonate rocks, and a near absence of shelly fossils, are generally interpreted to indicate a relatively deep-water depositional environment. the western assemblage also differs from the eastern assemblage in its components of bedded chert, basalt bodies, barite deposits, and sulfide deposits. the nature of the two assemblages and their relation to one another are critical for an understanding of the antler orogeny. the western facies assemblage is generally thought to be displaced from the west and to constitute the upper plate of an extensive thrust fault—the roberts mountains thrust. the eastern facies assemblage is thought to extend westward under the thrust plate. the basis for this belief is that the western facies domain is dotted with anomalous blocky exposures of contemporaneous eastern facies shelf strata, some of mountain size, surrounded by exposures of western facies rocks. these have been interpreted almost universally as exposures of the carbonate shelf facies in windows of the roberts mountains thrust sheet, and to prove the existence of that thrust sheet.