Dr. Mac Culloch on the Geology of Glen Tilt, 325 



three years ago, the same circumstance is described, and I may 

 here repeat that the granite in question is not decomposed, and 

 contains a very large proportion of flesh coloured felspar, the 

 quartz and mica being in small quantities. As granite is a rare 

 rock in Scotland, I have not been able to extend my observations 

 on this substance far, but I have also found the same magnetic 

 power in some of the granites which form the Ross cf Mull. 

 The porphyries also occasionally possess the same power. I have 

 observed it in different places, but most remarkably in those of 

 Cruachan, and expect that it will be found most conspicuous in 

 those, which from their black colour contain much hornblende 

 and approach to trap in their composition : it is not always that 

 circumstances allow of the repetition of these observations. I 

 have not often discovered it among the primary schists, but I 

 must add at the same time that my trials on those rocks have 

 been but few. Yet it occurs among the schists in the hills which 

 border Glen Tilt in more situations than one, although seldom in 

 an energetic degree. It is not limited to the hornblende slate, but 

 is found in those specimens of argillaceous schist into which that 

 mineral does not enter. I have also observed that some of the 

 beds of dark blue limestone exert a disturbing force on the 

 needle, and imagine that I found it greatest in a stratum which 

 crosses the upper part of Glen Fernat at the foot of Cairn Ree. 

 I have however examined limestones of the same apparent nature 

 in other parts of Scotland without having discovered a similar 

 property in them, so that I conclude it will be found but rarely 

 either among the rocks of this family, or the schistose ones. 



It has so long been known and so often remarked as a quality 

 common to the rocks of the trap family, that instances of it must 

 have occurred to every geologist. In these indeed it is often so 



