The Oldest known Rocks of the Earth's Crust

Abstract

MAY I welcome Prof. A. P. Coleman's letter on “Geology and the Nebular Theory” in NATURE for June 17, p. 775? It must be admitted that the achievement of A. C. Lawson at Rainy Lake in 1887, the elucidation by Sederholm of the floor of Finland, and the illuminating work of Canadian geologists, including Coleman, Adams, and Barlow, on the Grenville Series, have been slow in penetrating academic circles in the British Isles. The doorways were almost closed against them, and against the views of French geologists also, by the dead-weight of theories of dynamic metamorphism. Yet our confidence in a fundamental “Lewisian” gneiss was well shaken thirty years ago by Sir A. Geikie's announcement that this rock penetrated a sedimentary series (see A. Geikie, “Text-book of Geology,” 4th ed., vol. 2, p. 890); and a more detailed acquaintance with the ground would have led the same observer to withdraw his statement (ibid., p. 895) as to a “violent unconformability” between gneisses and Dabradian sediments in north-west Ireland. Some of us have lost no opportunity of comparing the conditions in our homeland, with those of broader Archaean areas. But even in our narrow lands, as I have ventured to urge from 1900 onwards, the teaching of the rocks themselves is unmistakable. The oldest known rocks are sediments, and the streaky structure of our ancient gneisses again and again records the stratification of ordinary sediments invaded by a granite magma.