'92 Prof. E. Hull. On a possible [May 16, 



May 16, 1889. 



Professor Gr. Gr. STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. " On a possible Geological Origin of Terrestrial Magnetism." 

 By Edward Hull, M.A., LL.D., F.R.S., Director of the 

 Geological Survey of Ireland. Received April 30, 1889. 



(Abstract.) 



The author commenced by pointing out that the origin and cause 

 of terrestrial magnetism were still subjects of controversy amongst 

 physicists, and this paper was intended to show that the earth itself 

 contains within its crust a source to which these phenomena may be 

 traced, as hinted at by Gilbert, Biot, and others ; though owing to 

 the want of evidence regarding the physical structure of our globe 

 in the time of these observers, they were unable to identify the 

 supposed earth's internal magnet. 



The author then proceeded to show cause for believing that there 

 exists beneath the crust an outer and inner envelope or " magma," 

 the former less dense and highly silicated, the latter basic and rich 

 in magnetic iron-ore. This view was in accordance with those of 

 Durocher, Prestwich, Fisher, and many other geologists. The com- 

 position of this inner magma, and the condition in which the mag- 

 netic iron-ore exists were then discussed, and it was shown that it 

 probably exists under the form of numerous small crystals with a 

 polar arrangement. Each little crystal being itself a magnet, and 

 having crystallised out from the magma while this latter was in a 

 viscous condition, the crystalline grains would necessarily assume a 

 polar arrangement which would be one of equilibrium. Basalt might 

 be taken as the typical rock of this magma. 



The thickness and depth of the magnetic magma beneath the 

 surface of the globe were then discussed, and while admitting that it 

 was impossible to come to any close determination on these points owing 

 to our ignorance of the relative effects of increasing temperature and 

 pressure, it was assumed tentatively that the outer surface of the 

 effective magnetic magma might be at an average depth of about 100 

 miles, and the thickness about 25 or 30 miles. The proportion of mag- 



