The Miocene of the Grenoble region (France) is composed almost entirely of conglomerates of deltaic origin. Among the pebbles are volcanic rocks of a rhyolitic nature, which have been studied microscopically and chemically. These rhyolitic pebbles may be the remains of a lower Tertiary volcanic episode with a center located in the ultra-Dauphinois flysch zone, now obliterated by erosion. For the western Alps, lower Tertiary volcanism would be considered an important link in the magmatic evolution connected with the Alpine orogeny.