OWING to my absence from home, I have only just seen the letters of the Rev. E. Hill, M.M.S., and James Heelis, in NATURE of August 2 and 9, on the above subject. The writers have apparently overlooked the very interesting report by Profs. Rucker and Thorpe, published in the Brit. Assoc. Report for 1889, p. 586, in which it is shown that “all the principal masses of basalt in the kingdom form centres of magnetic attraction,” and that “the Malvern Hills, though composed of diorite in which magnetic polarity can barely be detected, produce deviations of twenty minutes of arc at a distance of one mile from their axis.”