326 Notices of Memoirs—Prof. Hull on Terrestrial Magnetism. 
myself to Megalosaurus in the ‘Cat. Foss. Rept. Brit. Mus.’ p. 167, 
under the name of M. Dunkeri. JI am induced to make this new 
species because metatarsals obtained by Mr. C. Dawson from the 
Wadhurst Clay of Hastings are clearly specifically distinct from the 
above-mentioned specimen, and I provisionally refer them, on account 
of their larger size, to M. Dunkeri. 'The type of Ml. Oweni appears to 
belong to the right side, instead of to the left, as stated by its 
describer. 
NOTICES OF MEMOTRS. 
I.—On a PossiBLE Grotoeicat Ortern or TerResTRIAL Macenerism.! 
By Professor Epwarp Hutu, M.A., LL.D., F.R.S., Director of 
the Geological Survey of Ireland. 
ee 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 there is 
cause for believing the earth itself contains within its crust a source 
to which magnetic 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 observed that in the opinion of many geologists 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 the views of 
Durocher, Prestwich, Fisher, and many others. The composition of 
this inner magma, and the condition in which the magnetic iron-ore 
exists, were then discussed, and it was shown that it probably occurs 
under the form of numerous small crystals, with a polar arrange- 
ment; each little crystal being itself a magnet, and having crystal- 
lized 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 sur- 
face 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 tempera- 
ture 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 pro- 
portion of magnetic iron-ore in basaltic rocks was then considered, 
and it was shown that an average of 10 to 15 per cent. would 
express these proportions ; and assuming similar proportions to exist 
in the earth’s magnetic magma, we should then have an effective 
terrestrial magnet of from 24 to 3 miles in thickness. The thick- 
uess, however, might be very much greater than here suggested. 
1 Read at the Royal Society, May 16, 1889. 
