The Primitive Crust of the Earth

Abstract

IN reference to the letter of Dr. Harold Jeffreys (NATURE, July 29, 1922), I wish at once to say that nothing in my letter published on July 8 was intended to express my adhesion or non-adhesion to those who support the planetesimal hypothesis. Even if we think that the earth originated in a rain and concentration of solid planetesimals, we may, with Prof. R. A. Daly, regard its complete fusion at a later stage as a very probable event. At some time or other, the earth may well have possessed a crust consolidated from “igneous” fusion. Prof. J. Joly now suggests to us, with his unfailing brilliance of outlook, the recurrence of such a crust after successive meltings of the globe. What I have urged, however, is that the oldest rocks traceable by geologists must not be regarded as a record of a primitive crust. They are sediments, invaded again and again by igneous matter from below. We cannot conclude from our Archæan schists, which are so often converted into composite gneiss, that there was ever a crust formed of crystalline rocks about the globe. The “extent of the crust accessible to geologists” is, of course, much more than the film 2.5 km. thick stated by Dr. Harold Jeffreys. Owing to the great movements that bring up antique masses from the depths, rocks that consolidated finally under several miles of sediments now form a large part of the surface. But so far no planetesimal sediment has come to light, although matter of the mineral composition demanded by the hypothesis is associated with many igneous upwellings.