Abstract

Further studies of the paleomagnetic directions of red sandstones and siltstones of various geological ages in the United States are described. Usually the directions of magnetization of samples from a formation at one site are grouped symmetrically about a mean direction. From such a mean direction the position of the pole for that geological age can be calculated. There are, however, magnetically unstable formations in which the directions of magnetization are distributed approximately in the plane containing the present dipole field at the site and the original direction of the magnetic field. This planar distribution is the result of a superposition of a secondary magnetization on the original one. The former is thought to be a viscous or chemical magnetization acquired in the last 1,000–1,000,000 years.

Pole positions calculated from mean directions at different sites are consistent for the same formation and for different formations of the same geological age.

The study confirms the general trend of the polar-wandering curve for North America obtained by Runcorn (1956a), which lies around the northern Pacific Ocean: the pole being in the central tropical Pacific in late Precambrian time, moving across to the tropical western Pacific in the early Paleozoic and to Asia in the late Paleozoic and early Mesozoic. The data also show that the polar-wandering curve for North America is displaced westward relative to that for Europe, as Runcorn (1956b) showed, and provide an estimate for the amount of drift between the two continents since Mesozoic time, which is of the order of 30° in longitude.

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