Geologic time is one of the great definite quantities of natural science. The measurement and subdivision of that quantity, however, introduces human error. Even in the type Tertiary section of Europe the absolute time span represented in a single epoch is difficult to establish. On the Pacific Coast further difficulties are introduced by errors in correlation by means of vertebrate, invertebrate, and micro fossils and by orogenies. Formations lying near

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epoch boundaries are necessarily more ambiguous stratigraphically than others.

In California where marine and non-marine formations are intercalated, discrepancy in correlation has sometimes appeared, the vertebrate determination being higher than the invertebrate. The divergence is accentuated in the middle Tertiary by some vertebrate paleontologists who arbitrarily state that the appearance of the horse genus Hipparion introduced the Pliocene. Application of this criterion would place the Puente, Mint Canyon, and Modelo in the Pliocene. The writer adheres to his correlation table published in 1930 (Carnegie Inst. Washington Pub. 404, p. 85) placing these formations in the Upper Miocene not only on the basis of invertebrate stratigraphy but also because of morphologic and time relationships of Hipparion in Europe, Asia, and North America.

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