MAGNETICAL OBSERVATIONS IN SCOTLAND. 103 



the differences undei* the two first heads ; and that we are war- 

 ranted in considering such differences as evidencing real irre- 

 gularities in the magnetic direction at the respective stations, 

 caused by the presence of igneous rocks. We shall subsequently 

 find that this inference is confirmed by the agreement of the 

 intensities deduced by the horizontal and statical methods, 

 when the dij)S actually observed are employed in the reduction 

 of the horizontal vibrations, and in their extreme disagreement 

 when the dips due simply to the geographical positions are em- 

 ployed. 



A question here arises, how far the general results which we 

 have derived, in regard to the isoclinal lines, from the combina- 

 tion of the dip observations, are likely to have been affected by 

 these local irregularities ; and it is satisfactoiy in this view to 

 find, that a careful consideration of the errors in Table III. leads 

 to the inference, that the disturbing cause, whatever it may be, 

 has no uniform tendency, but that its effect is nearly as often 

 to diminish as to increase the dip. It is indeed a consequence 

 of the method of combination that the + and — errors should 

 nearly balance ; but had the effect of the disturbance at the 

 igneous stations been uniformly to augment the dip (for in- 

 stance), the sedimentary stations would all have appeared in 

 defect, and all the igneous ones in excess ; whereas the I'esults 

 at the sedimentary stations are indiscriminately in excess and 

 in defect, but to a very inconsiderable amount ; and at the ig- 

 neous stations thej"^ are also indiscriminately in excess and in 

 defect, but with differences of considerable amount. 



After much consideration, it does not appear to me that a 

 more satisfactory or probable conclusion would be arrived at, 

 were any one or more of the stations now included in the cal- 

 culation; to be withdrawn from it. Every observation of the 

 dip has been incKided in the calculation, excepting two. One 

 of these was at Oban, where I hastily observed the dip on a 

 trap rock at no great distance from the wharf, whilst waiting 

 for a steamer, and the result has been found to differ more than 

 a degree from the dip assigned by calculation. I suspected the 

 locality whilst making the observation ; and had time per- 

 mitted I should have removed the instrument to another spot, 

 and repeated the observation. As it is I can only consider it 

 too doubtful a result to be placed on the same footing with the 

 others. The other rejected observation was of a very extraor- 

 dinary nature. On a rock on which I landed, on the west side 

 of the harbour at Loch Scavig, I observed a dip of 78° 10'*3, 

 exceeding by !j° that which could be assigned to tlie geographical 

 position. I had never before experienced an irregularity of dip 



