302 MR. A. W. RiTCKER AND DR. T. E. THORPE ON A MAGNETIC 
relations uncertain, as the direction of the Horizontal Force would lie nearly along 
the valley line. The direction deduced from Welsh’s observation would place it in 
the next district. The same remark applies, though more doubtfully, to Ardrishaig. 
If, however, these three or four border stations be put aside, the map leads to a 
consistent view of the magnetic state of the district, which is in exact accord with 
that previously arrived at. The ridge line lies in the region of greatest Vertical Force 
disturbance. The Horizontal Forces tend towards that region, and at points within 
it are, on the whole, directed to the ridge line. The discrepancies between our results 
and those of Mr. Welsh are hardly, if at all, greater than those between our own, 
when repeated. It is remarkable that the south-western stations indicate a centre of 
attraction out at sea, but our results are completely confirmed by Welsh’s at Glen- 
morven. We should certainly have expected a priori that Mull, which is highly 
basaltic, would be a centre of attraction. 
Between Elgin and Banff there is a strong local centre. Welsh observed the 
Horizontal Force at Banff only, but the Declination obtained by him at Elgin is less 
than that at Banff, the difference being about 10', as against 7' given by our survey. 
The twm sets of observations are thus in agreement. 
lice Scotch Coal-field District. 
This district is bounded on the north by the valley line which forms the southern 
boundary of that which has just been discussed. 
The southern boundary is inclined to the magnetic meridian at an angle which does 
not give any special advantage to either the Horizontal Force or the Declination as 
a means of determining its position. We have therefore taken the Declination line 
corrected (as described on p. 296) by Welsh’s observations at Makerstoun and Melrose. 
There is not a single abnormal station in this district, and the results of Welsh’s 
observations fit in very well wdth ours. At Ayr, indeed, there is a very considerable 
discrepancy, and the forces at Alford and Gretna are larger than we should have 
expected if they are regarded as the resultants of the attractions of the two regions, 
close to the boundaries of wdiich these stations are situated. 
All the four stations at which the Horizontal Force is a minimum, viz. (taking them 
in order from the west), Cumbrae, Bow, Stirling and Dundee, are situated close to the 
northern boundary of the Scotch Coal-field, in which there are large masses of basalt. 
The ridge line runs through the middle of this basaltic district and passes through 
or near two regions in wBich the disturbance of the Vertical Force is positive [i.e., in 
which the force is great) to the east and west respectively.- At Cumbrae the Vertical 
Force is a maximum, and the existence of a region of high Vertical Force in this 
neighbourhood is remarkably confirmed by the fact that Welsh’s observations give a 
positive disturbance at Ardrossan a few miles further south, though at the neighbour¬ 
ing stations both to the south and north it is negative (fig. 23). On the wdrole then 
