SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 641 
As the main object of our enquiry has been to discover regional attractions, we 
have for the most part given a wide berth to dykes. A chain of stations was, how¬ 
ever, run across the Boulmer Dyke, whiclr is exposed only on tlie coast, and runs out 
to sea in an east and west direction. Observations were taken above it a few yards 
inland, where it is believed to be covered by about 25 feet of drift, at a point 90 yards 
to the south, and at two points 70 atid 90 yards to the north of it respectively. 
The Disturbances were very small, and indicated, if anything, a slight repulsive 
force. It would not be surprising if no Disturbance were detected at 70 yards ; it is 
somewhat remarkable that no result was obtained immediately above the trap. 
There is, of course, no lack of evidence that dykes do produce magnetic Disturbances, 
and the cause of our failure to detect them in this case must be due to some special 
circumstance. 
As we have already stated, the mean susceptibility of the four specimens of the 
I'ock which we have examined was 0*0015, but the fragments differed so wddelv (from 
0‘00361 to 0*00003) that it is possible the true average susceptibility may be much 
less. It is also possible that near the spot where the observations were made the 
dyke is permanently magnetized iii a direction opposite to the induced magnetization 
produced by the earth’s held. Such hypotheses are, liowever, mere guesses. The 
point requires further elucidation, but as it can be taken up at any time, while 
regional forces can only be studied near to the epoch for which tlje Terrestrial Iso¬ 
magnetics have been determined, we have not up to the present pursued it further. 
As we are discussing observations made close to masses of basalt, this is, 
perhaps, the most convenient place to describe two sets of experiments made near 
the igneous rocks to the south-east of Limerick. 
A little to the north of the village of Kilteely a large portion of a curious ring- 
shaped mass of basalt is exposed. It is several miles long and from half to three- 
quarters of a mile broad, and is the outcrop of a basin of basic rock. 
The results obtained are summarized belovq the Disturbing Forces being, as usual, 
expressed in terms of 0*00001 C.G.S., or 0*0001 metric unit. 
Kilteely. 
N umber 
of 
station. 
Description of station. 
Disturbing Forces. 
Horizontal. 
Vertical. 
807 
About 300 yards S. of the basalt . . . 
19 
■L125 
806 
Close to the S. edge of the basalt . . . 
659 
-h 37 
808 
Close to the N. edge of the basalt . 
510 
- 17 
809 
About 120 yards N. of the basalt . . . 
70 
-235 
810 
About 300 yai'ds N. of the basalt . 
115 
-h 7 
4 n 
MDCCCXCVI.—A. 
