The magnetic properties of three carboniferous sills

An account is given of palaeomagnetie measurements on borehole specimens from three dolerite sills of Carboniferous age. The magnetic directions were closely grouped for one sill but widely scattered for the other two, showing that contrary to the experience of previous workers, specimens from underground do not always yield satisfactory palaeomagnetic results. The intensities of magnetization were very low at the margin of each sill. In one instance this phenomenon was studied in detail. It seems to be due to the fact that at the margins, the iron occurred mainly, in the form of a nonmagnetic carbonate, produced when the sill was intruded into limestones. Some wider implications of the work are discussed.