A FEW years ago Mr. Geo. L. Harrington, of Buenos Aires, was prospecting in the Santa Barbara district of the Province of Jujuy, Argentina. In this region we find deep gulches with good exposures of the geological formations. A very conspicuous and important formation consists of green rock, easily breaking up into small angular fragments, commonly with conchoidal fracture. In its upper part it shows streaks of red, increasing until we reach an entirely red deposit. We interpret this as indicating increasing aridity, culminating in desert conditions with wind-blown sands. The red rock appears to be without fossils, and above it is a great mass (possibly 2000 feet) of yellowish deposit of more recent date. Dr. G. Bonarelli (1921) placed the green rock in the upper division of his Calcareo-Dolomitic horizon, but owing to the absence of fossils the age remained unknown. Mr. Harrington, following up the gulch which runs west from Sunchal, came upon a little deposit of fossil insects, and collected a number of specimens. Most of these were eventually transmitted to the U.S. National Museum, whence they were sent to me for study. They were found to consist of seven species of beetles and a caddis fly, almost certainly of Tertiary age. The horizon, was in the typical portion of the green formation, where it is interbedded at intervals with heavy limestone.