Dr. Mac Culloch on the Geology of Glen Tilt, 327 



and partly of that syenite which appears to belong to the same 

 family, being, like the clinkstone, superimposed on the secondary 

 strata. Here the blue clinkstone possesses the magnetic virtue 

 in the most eminent degree, but it is also attached to the syenite, 

 as well as to the grey porphyries which are derived from it by the 

 same imperceptible gradation as are the porphyritic varieties of 

 clinkstone from the simple ones. 



The magnetic property of a rock of serpentine was remarked by 

 Humboldt some time ago, and I have also found it in this rock in 

 the only situations where I have as yet had an opportunity of ex- 

 amining it, namely at Portsoy, and in the island of Scalpa.* In 

 both these places the magnetic power is considerable, and in some 

 particular spots nearly as great as in the most striking examples 

 among the trap rocks. In hand specimens I have also found that 

 the serpentine of Anglesea, and that known by the name of Egyptian 

 green, exert considerable influence on the needle, so that the 

 property is in all probability generally dispersed among the rocks 

 of this description, as it is among the traps. I may add finally, 

 that I have observed it, but sparingly and feebly, among some of 

 the dark pitchstones, and that in the augite rock which I have 

 formerly described as forming so large a part of the island of Rum ; 

 it is as vigorous and common as among the traps with which that 

 rock has so strong an affinity both in mineralogical and geological 

 character. 



Such then is the catalogue of the rocks among which I have 

 found the property of disturbing the magnetic needle by the exer- 

 tion of their own magnetic powers, and it will be seen that they 

 occupy a considerable share of those rocks which constitute the 



* To distinguish this Scalpa from other islands of the same name, I must say that it li^ 

 at the eastern side of Harris, and is renurkablc for its lighthouse. 



