SURVEY OP THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1891. 
largest basic masses occur. This is unmistakably a centre of attraction. The 
Horizontal Disturbing Forces at Beaumaris, Carnarvon, Pwllheli, Port Macloc, 
Abergwynolwyn, Bala, Bettws-y-Coed, and Llandudno are all directed towards the 
igneous rocks, which are in part basic, and may in part rest on a basic substratum. 
Following next in importance after North Wales come Devonshire and Cornwall, 
Wexford and Wicklow. In both of these the basic masses occur in long lines, which 
suggest that they have not much lateral extension, or that they are the edges of 
relatively thin sheets, or are connected with narrow pipes. However this may be, 
though there are suggestions of lines of magnetic attraction parallel to both of them, 
in Devonshire and in the east of Wexford, neither is a centre of attraction. There is 
a station of relatively high Vertical Force on the serpentine of the Lizard, but here, 
too, the main centre of attraction appears to be out at sea. 
Three other districts remain. The most important is North-West Donegal, a wild 
region, in which we have comparatively few stations. At five stations near London¬ 
derry the Horizontal Disturbing Force is directed towards the trap rocks. The same 
statement is true of the stations to the north of the basic masses near Newry and 
Caiiingford. To the south of these the main attraction is exerted towards a ridge 
line which runs out from them, and probably marks an underground continuation of 
the trap. 
In Pembrokeshire, also, the strips of igneous rocks lie close to a ridge line, and the 
Horizontal Disturbing Forces at Cardigan, Haverfordwest, and Milford are directed 
towards it. 
The basalt to the south-east of Limerick also lies near a rido-e line, but is not a 
centre of attraction, though some of the igneous rocks are very ba,sic. 
The scattered masses in Mid-Wales and Mid-England, and the dykes in the north 
of England, are discussed separately. 
Putting them aside the results we have arrived at may be summed up as in the 
following Table ; — 
Principal Masses of Trap Bocks in the order of their importance. 
Skye and Mull. 
Probably traversed by ridge lines 
Antrim. 
A centre of attraction 
Mid-Scotland. 
North Wales. 
Wexford and Wicklow . 
No apparent regional magnetic effect 
Devonshire and Cornwall . 
,, _ „ „ (except 
at the Lizard) 
Donegal. 
A centre of attraction 
Pembrokesliire . . . . . 
On a ridge line 
Limerick. 
Near a ridge line 
Igneous Rocks ivhich are hut slightly, or not at all Basic. 
. In general, masses of non-basic igneous rocks appear to produce little or no effect on 
the needle. This holds good of Dartmoor, Wicklow, and many parts of Scotland. 
