652 MR. A. W. RtiCKER AND DR T. E. THORPE ON A MAGNETIC 
Obviously all difficulties are increased if a magnetic district is largely covered 
by the sea. 
There are three principal districts in which basic rocks are found, with a profusion 
far exceeding that which occurs in any other part of the United Kingdom. They 
are Antrim, Mid-Scotland, and the West Coast of Scotland. 
The first two of these unquestionably attract the needle at places near to but clear 
of the mao-netic rocks. 
o 
io prove this we need only refer to Map 14, in which the basic rocks are coloured 
red, and the directions of the Horizontal Disturbing Forces at stations near the 
rock are shown. All round the Antrim basalt the Horizontal Disturbing Forces are 
directed towards it. This is shown at no less than fourteen places, viz., Drumsurn, 
Dungiven, Sperrin, Dungannon, Armagh, Lurgan, Lisburn, Belfast, Carrickfergus, 
Larne, Ballygalley Head, Carnlough, Waterfoot, and Bally castle. 
At Draperstown the Horizontal Disturbance is directed away from the basalt, 
in which, bo'wever, this station is to a certain extent embayed. It therefore hardly 
satisfies the condition we are laying down ; viz., that the stations considered must be 
clear of the attracting mass. 
Similarly, a line of stations can be drawn round the Scotch Coal Field District, which 
indicate that it, too, is a centre of attraction. They are Campbelton, Torrlsdale, 
Loch Banza, Cumbrae, Bow, Dumblane, Crieff Junction, Perth, Stanley Junction, 
Glamis, St. Andrews, Dunbar, Heriot, Dolphinton, Carstairs, Abington, Loch Doon, 
Pinvalley, and Penwherry. 
Against these nineteen must be set Leadburn on the south, and Alyth and Monikie 
in the north. In the latter two cases, however, the apparent discrepancy is readily 
explained. There is abundant evidence that another ridge line runs a little- to the 
north of the district we are considering, which affects Alyth. 
The results at Monikie and Arbroath, on the other hand, suggest an underground 
extension of the volcanic rocks.* However this may be, the number of discrepancies 
is relatively very small. 
On the West Coast there is not the same clear evidence that the visible masses are 
centres of attraction. We have shown that the hypothesis that ridge lines run 
through Skye and Mull is consistent with the fircts, but the effects of the visible 
masses a})pear to be of less importance than those due to concealed centres. In Skye 
there is a powerful focus of attraction on the eastern portion of the island, where the 
surface rock is not basalt; and, as we have pointed out, the effect produced by Mull 
is subordinate to an attraction directed to the south of the Hebrides. 
It must, however, be recollected that much of this district is covered by cue sea, 
and that important ridge lines which run near its boundaries make it very difficult to 
solve all the problems it suggests. 
Next to the distiicts above mentioned, North Wales is, perhaps, that in which the 
* lu Forfarshire another volcanic area occurs, belonging to the Old Red Sandstone. 
