Is the principal source of the secular variation of the Earth's magnetism within or without the Earth's crust?

In the papers on the “Distribution and the Secular Variation of the Earth's Magnetism,” presented before the Washington Philosophical Society in May, 1895, I showed that certain laws pertaining to the distribution of the magnetic elements over the earth's surface are similar to those pertaining to the secular variation as derived thus far. I shall refer to four of the deductions then made, as they will be found interesting in connection with some of the conclusions reached in this paper. These laws 2 are: I. The minimum range in declination along a parallel of latitude occurs near the equator, generally increasing upon leaving the equator. II. The maximum range in inclination along a parallel of latitude occurs near the equator, generally diminishing upon leaving the equator. III. The minimum average secular change in declination along a parallel of latitude from 1780 to 1885 occurred near the equator, the values generally increasing upon leaving the equator. IV. The maximum average secular change in inclination along a parallel of latitude from 1780 to 1885 occurred near the equator, the values generally diminishing upon leaving the equator.