648 
MR. A. ^y. ruck:p]r and dr. t. p:. thorpe on a magnetic 
It is immediately surrounded by a narrow band of the coal measures, which are 
succeeded bv the Old Red Sandstone. 
1/ 
Observations were made on the east and west sides at Bitterley and Hopton 
Wafers, al points about a mile and a half from the basalt. 
The result was that the Horizontal Disturbing Force at Bitterley acted nearly due 
west, i.e., at right angles to the line joining the station to the centre of the basalt. 
Hence, no relation whatever v'as established between the directions of the Disturbing 
Forces and the magnetic rocks. 
This result is in conformity with the above calculations, but it may be observed it 
has more than a merelv negative value. If a relatively thin sheet of basalt were 
extruded through pipes communicating with a deep-seated magma, it is conceivable 
that in some cases the position of the outflow may have been determined by the 
nearer approach of the magma to the surface. 
If this were the case, and if the top of the concealed underground basaltic fold 
were at a moderate depth, it is possible that attractions in play in the neighbourhood 
of the visible magnetic mass, which could not be due to it, might be caused by its 
concealed foundations. Thus the absence of attractive forces a mile and a half from 
Titterstone Hill proves that there is beneath it no very remarkable pillar-like 
prominence on the basic substratum. 
A number of isolated patches of “ gveenstone ” occur near the boundary between 
the Devonian and the Carboniferous formations on the west of Devonshire. 
To the north of the larger of these patches, and about a mile and a half distant 
from them, is Brent Tor, an isolated hill with a basic core. Observations were made 
at two stations to the east and west of it respectively. In the direct line they were 
a little more than half a mile apart, on good observing ground, and each about an 
eighth of a mile from the Greenstone. 
The directions of the Disturbing Forces were 30° W. of N. at the westerly 
station (at which the Disturbing Force was much the larger), and 26° W. of X. 
at the easterly station. Hence both Forces were directed away from the relatively 
large quantities of Greenstone to the north, and the much nearer, but also the much 
smaller mass of Brent Tor, produced no appreciable effect. 
A mass of isolated basalt and andesite occurs at Builth. 
The Disturbing Force at a station a mile to the north points directly awa}" from it 
towards the important magnetic ridge line which occurs to the south. 
To the south of the estuary at Barmouth is a mass of felspathic trap and “ green¬ 
stone,” about 4 miles wide. Igneous rocks are also found to the north of the town, 
and, indeed, occupy a large part of North Wales, but the largest mass in its 
immediate neighbourhood is Cader Idris to the south. 
Two stations were carefully chosen on good observing ground about 3^ miles apart, 
and from a mile to a mile and a half distant from the Cader Idris range. In both 
cases the Disturbing Forces pointed north, thereby showing that the regional 
