Introductory Statement

During the summer of 1917, while studying the Tertiary formations on the north side of Mount Diablo, in the Mount Diablo quadrangle, the writer noticed a marked difference in strike between certain of the beds which up to this time had been considered a part of the Tejon (Upper Eocene). This discordance, a difference of nearly 50 degrees, meant one of two things: either it was due to faulting or to an unconformity. Later detailed work has shown conclusively that here we have an unconformity, which was the result of crustal movements of considerable magnitude.

Briefly stated, it is the writer’s conclusion, after studying the unconformity mentioned above, together with fairly large collections of fossil invertebrates from both above and below the line of contact, that we have here the evidence of a structural break of more than local importance; that the fauna found above the unconformity in beds associated . . .

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