Rhyolite flows, felsic ash-flow tuffs and volcaniclastic rocks of the Pony Trail Group (Muffler, 1964) are the only recognized Jurassic felsic volcanic rocks in Nevada, cropping out in northern Eureka County in the Cortez Mountains area. They are dated as Jurassic by a radiometric date from 1972 (Smith and Ketner, 1976). The Pony Trail Group is made up of (in ascending order) the volcaniclastic Big Pole Formation; a silicic ash-flow tuff unit, the Sod House Tuff; and the boldly outcropping Frenchie Creek Rhyolite made of tuffs, volcaniclastic horizons and flows (Smith and Ketner, 1976). While some of these rocks likely are Jurassic, rocks mapped on the Elko County map as the Frenchie Creek Rhyolite exposed in the Elko Hills northeast of Elko have been shown to be Tertiary and renamed (Ketner, 1990) so it is possible that parts of the section included in the Frenchie Creek Rhyolite are not Jurassic. This unit corresponds to unit Jv on the 1978 State map.