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influenced by it in their distribution. Only two of these, the middle and the lower, need here be considered. The middle formation is a tuff which has already been called the Tuscan tuff. Below the Tuscan tuff and above the Cretaceous are gravels, sands, and clays, which apparently occupy the exact taxonomic position of the Ione formation of Becker, Lindgren,^[1] and Turner, and may therefore be appropriately designated by the same name.

Tuscan tuff.—The Tuscan tuff is composed wholly of volcanic material. It will be considered first, for the reason that it can be most easily identified in different localities, and can be used to great advantage as a reference plane in considering the Ione.

On the western border of the Sacramento valley the most southern exposure yet observed is on Thomes creek, four miles east of Paskenta. From this point it has been traced with varying thickness for fifty miles across all the streams, cutting the eastern margin of the baselevel from Elder creek to Redding. It continues, with interruptions, around the northern end of the Sacramento valley to the thick deposits of similar material in the Lassen Peak region. It thins out to the westward and laps over on the baselevel in such a way as to indicate that the baselevel was formed before the great volcanic eruption which gave birth to the tuff.

Ione formation.—Beneath the Tuscan formation lies the Ione, which rests upon the upturned and eroded edges of the Cretaceous (Shasta-Chico) strata with conspicuous unconformity. In the Bald Hills region, northeast of Paskenta, it is composed of clay, and thins out rapidly to the westward against the edge of the baselevel. Farther northward the formation thickens somewhat, and contains much gravel, but everywhere it thins out rapidly to the edge of the baselevel. In the Lassen Peak region, beneath the lava, it has its greatest development, and is many hundreds of feet in thickness. To the northeastward it borders