6 



T. E. Thorpe and A. W. Riicker. 



[Nov. 15, 



not only the general accuracy of Mr. Welsh's observations on this 

 particular point, but also Professor Stewart's inference as to the 

 probable locale of the area of disturbance. 



Since the date of the publication of Professor Stewart's Report, 

 the Island of Mull, and the adjacent district of Morvern, have been 

 made the subject of very careful geological and petrological study, 

 more particularly by Professor Judd (see his paper in the " Quart. 

 Journ. Greol. Soc," xxx, 220), who has demonstrated that the great 

 central mountain group in the island is the denuded core of an 

 immense volcanic pile, the base of which must have had a circum- 

 ference of at least forty miles, and the lava streams from which must 

 have flowed to enormous distances and in quantities so vast as to have 

 accumulated to a thickness of 2,000 feet. We have in these huge 

 accumulations of gabbro, dolerite, basalt, tachylite, &c — all rocks 

 containing more or less magnetite and, possibly, even finely-divided 

 metallic iron, as in the basalts of the north of Ireland and of Green- 

 land, an undoubted source of local attraction, sufficient in all pro- 

 bability to affect the needle to the extent observed by Mr. Welsh. 



It seemed to us highly desirable, however, to repeat Mr. Welsh's 

 observations in the light afforded by Mr. Judd's analysis of the 

 geological features of the district. Although the geological facts 

 doubtless serve to strengthen the validity of the magnetic observa- 

 tions, it should be stated that the direct evidence as to the existence 

 of the great area of local attraction practically rested upon a single set 

 of observations. It appeared to us, therefore, desirable, as a prelimi- 

 nary step to any subsequent survey of the west coast of Scotland, 

 to confirm, in the first place, Mr. Welsh's observations as to the 

 existence of the local attraction ; secondly, to ascertain, if possible, 

 the position of maximum disturbance ; and thirdly, to determine the 

 influence of the area on the direction of the isoclinal line. 



Our observations were made with an excellent dip-circle belonging 

 to Owens College, for the loan of which we are indebted to Professor 

 Balfour Stewart. The instrument is known as " Dover No. 6 " ; it 

 had already been employed by one of us in magnetic observations in 

 America and in the Azores (see " A Magnetic Survey of the Fortieth 

 Parallel in North America," " Proc. Roy. Soc," vol. 30, 132; and 

 "Note on the Magnetic Inclination in the Azores," "Proc. Roy. 

 Soc," vol. 31, 237). The circle is provided with two needles, each 

 3 J inches long and 0'27 inch in maximum width. Similar precautions 

 were taken to preserve the needles from rust as are indicated in the 

 communications referred to, and the method of observation was the 

 same as that previously adopted ; it is identical with that described 

 by Mr. Welsh in the " Admiralty Manual of Scientific Inquiry," 

 third edition, 1859. With a single exception duplicate and indepen- 

 dent observations were made in all cases with the two needles. 



