Laboratory determinations of the magnetic properties of certain igneous rocks

Although the methods of magnetic survey in the field are now well estab­lished, there have been relatively few direct determinations of the magnetic properties of the rocks concerned. Conclusive evidence has now been obtained that some of the natural rock-masses are permanently magnetized ; this effect is superimposed upon the magnetization induced by the earth’s field, and in certain rocks it appears that even a small permanent magnetization may seriously modify the magnetic profiles. Experiments were therefore under­taken with the general object of linking up the evidence provided by field surveys with that obtainable in the laboratory. The material was chosen with a view to providing an experimental basis for the interpretation of the magnetic surveys that have been completed by the Geological Survey during the past few years. The collection and prepara­tion of the specimens were undertaken by the Survey, and the laboratory measurements have been carried out by Professor E. F. Herroun, to whom the author has been greatly indebted for criticism and advice throughout this work.