SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1891. 055 
The result, shown graphically in Map 14, is that a distinct attraction towards tlie 
Cheviots is exhibited at the five stations nearest to them. 
Tliccarton Junction and Alnwick, distalit about 10 and 12 miles respectively from 
the crystalline rocks, are apparently outside the range of their influence. Hawick, 
Selkirk, and Earlston, are affected, rather by the ridge line which runs from the 
igneous centre westwards, than by the visible basic rocks themselves. 
Considering how small the second residuals are, the largest northerly and westerly 
disturbances (apart from Jedburgh and Kelso) being 0-00042 and 0-00022 C.G.S. 
unit only, we think the result very satisfiictory. 
GaJivay. 
We have observed at six stations close to the borders of the ig’neous 
and gneissic rocks in Galway. They are Clifden, Ballinahinch, Maum Bridge, 
Oughterard, Galway and Spiddle. At Galway itself we confine ourselves for the 
present purpose to the result obtained at a station close to but not on the granite. 
Spiddle on the other hand is on the granite, and therefore hardly falls within the 
rule we are following. As, however, it is upon the coast, we include it in Map 14. 
At all six stations the Horizontal Disturbing Forces point to the crystalline rocks, 
among which there are many basic intrusions. 
The attractions apparently exerted in the North of England and in Galway by 
rocks, which, taken as a whole, are but feebly magnetic, are, tre believe, capable of an 
explanation quite in accord with the recent tendency of opinion among geologists. 
It IS thought that in many cases the fluid magma out of which the rocks were formed 
was arranged in layers of various densities, the lighter non-basic materials floating on the 
heavier and more magnetic constituents. Where sucli a separation occurred a group of 
non-susceptible rocks would now stand upon a magnetic foundation. It is conceivable 
that investigations, such as our own, may help to determine where this is the case. 
Faults. 
It appears at first sight that the magnet would be affected if a fault in the super¬ 
ficial strata extends down to magnetic rocks. 
There are, however, a number of arguments which show that no noticeable disturb¬ 
ance might be produced. 
If the depth at which the magnetic rocks occur is, say, four or five times the throw 
of the fault, the magnetic effects would in general be very small. 
Even when this is not the case, the possible effects are more complicated than would 
at first be supposed. If the magnetic rocks form a horizontal slab, the Vertical Force 
due to the horizontal faces will be positive above the mass, but negative at parts near 
to but not over it. I he positive or negative Forces may be strengthened or may not 
be affected by the vertical face, according as the latter is perpendicular to the 
magnetic meridian and to the south of the slab, perpendicular to the magnetic meridian 
and to the north of the slab, or in the magnetic meridian, 
