SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 649 
attraction was predominant even a mile and a half from a very conspicuous basaltic 
mass. 
Limerick is near the northern boundary of an isolated series of patches of igneous 
rocks, but the Horizontal Disturbing Force acts northwards, i.e., away trom tlie 
intrusive masses.*'" 
Conclusions such as these are important, as showing that even within moderate 
distances of relatively large ma.sses of basalt the regional forces may be determined 
free of all purely local disturbance. 
Basaltic Districts. 
We now turn to the apparently inconsistent deduction that, in spite of this fact, 
magnetic rocks may account for ridge lines, which appear to dominate very large 
areas. 
In such cases we have to assume (1) that the vertical thickness of the mass is 
much greater than has hitherto been supposed, and (2) that it extends laterally 
underground to great distances from the points wliere it occurs on the surface. 
These assumptions must, we think, appear reasonable, though, in some cases, no 
known geological hrcts may warrant them. 
It must, however, be I’emembered that the knowledge of geologists is practically 
confined to a few thousand feet from the surface, and in many cases does not extend 
so far. 
But the magnet would be capable of detecting large masses of magnetic rock at a 
depth of sevei'al miles, and the theory we are discussing can hardly be entertained 
unless we can assume that such masses exist. 
Nor is the hypothesis of the lateral extension of trap rocks for great distances 
around the points where they appear on the surface negatived l)y the fact that the 
sides of the visible mass are vertical, or that it is known to be a relatively thin sheet. 
The surface basalt may .spiing from a great underground told of magnetic matter, 
v/hich may slope gradually away from the area at which more or less accidental 
circumstances determined the form of the extruded mass. 
If, however, this is so, we must draw attention to the fact tliat these considera¬ 
tions reduce the significance of the particular place or places where the rocks appear. 
The highest point on an island may be at one end. Mountain chains do not culminate 
exactly in their centres. In like manner the point at which an underground mass of 
basalt reaches the surface may or may not lie on the magnetic ridge line which marks 
the points where the downward pull due to the whole mass is a maximum. This 
might explain the fact that near Limerick, and in several other places, masses of 
basalt lie near to but not on a ridge line. 
If, on the other hand, the exposed portion is very large and widespread, its 
* Ob.servations made close to these rocks are described below, pp. 641-2. 
MDCCCXCVI,-A. 4 0 
