The Sierra Nevada in the Light of Isostasy. By Andrew C. Lawson (pages 1691-1712)

Comment by Perry Byerly

In the November (1936) number of this journal A. C. Lawson1 concluded from isostatic considerations that the granitic mass of the Sierra in the vicinity of Mount Whitney is of a thickness about 68 km., its base extending down some 19 km. into the ultra basic rock of the earth's mantle. He has asked me to point out certain seismometric data that tend to confirm the great depth of penetration of the mountain mass.

A study of the Eureka (California) earthquake2 of June 6, 1932 first indicated that the Pn wave of the seismologist, when it originates in northern California, is delayed in its arrival at the Owens Valley seismographic station of Tinemaha. The Pn wave is a first preliminary (compression-rarefaction) wave which travels in the ultra basic rock below . . .

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