638 
MH A. W. RiiCKEE, AND DR. T. E. THORPE ON A MAGNETIC 
The magnetic rock theory is unquestionably open to the criticism that whereas it 
attributes Disturbances which extend for scores or even hundreds of miles to 
magnetic rocks, large masses of such rocks often produce little or no effect at 
what are regarded as surprisingly small distances. 
It is therefore desirable to apply calculation to a few cases, and to show that 
results, which are at first sight discordant, do not in any way invalidate the 
hypothesis. Some of them can hardly be said to support it, as the bases of the 
magnetic masses are concealed underground, and in the calculations, assumptions 
which are unproved, and are perhaps incapable of proof, have to be made as to the 
forms of these concealed foundations. All that can be done is to show that the 
different results are compatible with, and are therefore not valid evidence against, 
the rock theory. 
Two points made in the paper already referred to may be mentioned here, though 
they need not be further discussed. 
The first is that the purely local Disturbance, due to a mass of magnetic rock, of 
which the susceptibility is as small as that of basalt or gabbro, will be the same, 
whether it rests on magnetic or non-magnetic material, though of course in the 
one case it will, and in the other it will not be supplemented by other Disturbing 
Forces. 
The second conclusion is that, having regard to the rise of temperature as the 
depth increases, iron and magnetite vill cease to be magnetic at about 12 miles 
from the surface of the globe, if the great pressure to which they are there subjected 
does not affect their behaviour in this respect. In no case, therefore, can the 
lower face of a mamietic rock be assumed to be more than 12 miles from the surface. 
o 
Bearing these general conclusions in mind, we may now proceed to consider 
in detail whether any connection exists between local and regional magnetic Dis¬ 
turbances observed in the United Kingdom, and its geological confirmation. 
The discussion will be divided into five heads, viz., the relations of Magnetic 
Ridge Lines or centres of attraction to :— 
(1.) Dykes, basaltic sheets, and isolated masses of trap, the largest linear 
dimensions of which do not exceed a mile or two. 
(2.) Districts which are either entirely basaltic or in which numerous masses 
of trap appear. 
(3.) Non-basic masses of igneous rock. 
(4.) Faults. 
(5.) Regions in which the arrangement of the rocks is anticlinal. 
To illustrate our arguments Map 14 has been prepared for us under the kind 
superintendence of Professor Judd, F.R.S. 
The basic rocks are coloured red. The other formations are divided into groups 
in such a way as to bring out the points on which we wish to dwell. Only those 
