324 Dr. Mac Culloch on the Geology of Glen Tilt, 



openings at KilHcrankie, has hitherto proved insufficient to under- 

 mine and remove it ; in consequence as it would seem of the lateral 

 direction of its course produced by the fall of those portions of 

 alluvium w^hich have formed the small holm on which Fascally is 

 situated. Thus it is for the present protected from change, and 

 perhaps destined to remain, for a longer period than that in Glen 

 Tilt, a monument of those revolutions which point out an extensive 

 but transitory action of water on the surface of our globe. 



I have remarked in the preceding paper, that the granite which 

 forms the right boundary of Glen Tilt has the property of affecting 

 the magnetic needle. This influence is far from inconsiderable, 

 and in many cases produces not only remarkable local variations, 

 but a disturbance of that instrument so great as to render it useless 

 for the purposes of ascertaining any meridian whatever. It is not 

 limited to the syenitlc varieties, where from the predominance of 

 hornblende we might reasonably expect to find it more active, but is 

 equally to be found existing in those granites which do not contain 

 this ingredient. Neither does it depend apparently on the micaceous 

 ingredient, since in many of the rocks which shew it strongly the 

 mica is in very small proportion. It seems to be equally independent 

 of a state of decomposition in the granite, since it is here, as elsewhere, 

 to be found inherent in fresh specimens, although on the summit of 

 Goatfell in Arran, as I have remarked on a former occasion, it 

 appears most conspicuously in those specimens where the iron is 

 becoming carbonated and the rock is tending to disintegration. In 

 the paper to which I here allude,* I noticed this fact as being nearly 

 a solitary one at that time, but I have had occasion to observe it 

 since on various occasions. In a paper on Cruachan, drawn up 



* Geological Transaction^, vol. ii« 



