Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/335

 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 examining 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 extent ion of their own magnetic powers, and it will be seen that they occupy a considerable share of those rocks which constitute the