A field test of palaeomagnetic stability is described, based on a comparison of the magnetizations in mineralogically identical baked and unbaked sedimentary rocks. If successful, the test demonstrates the stability of the magnetization acquired during baking. Two instances are given of its actual application, one of which has led to a re-interpretation of the palaeomagnetism of certain Carboniferous formations.The test is compared with the three main field tests in common use.
I. IntroductionThis is an account of a new field test for the stability of magnetic directions in ancient rocks, which may supplement the three field tests already in use. The new test is based upon measurements on baked rocks. Many years ago, Brunhes (1906) pointed out that if the magnetic directions in associated baked and igneous rocks agreed, they could probably be regarded as stable. Our work is essentially an extension of this idea, for which measurements are made not only on the baked rock but also on neighbouring unbaked material. As a result one can eliminate the main weakness of Brunhes test, which is that occasionally both baked and igneous material can become simultaneously remagnetized after cooling by regional metamorphism.It will be most convenient to describe the test by means of two concrete examples: the first occurring in material associated with a Tertiary dyke, 40-50 million years old, from Port-na-Traigh-Linnhe near Oban, Argyllshire ; and the second in materi'al associated with the Carboniferous sill about 250 million years old that forms the Clee Hills in Shropshire.
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