THE TRAP ISLANDS. 79 



in many important particulars to granite, may suggest 

 the idea of a period of time more remote and more 

 nearly corresponding with that in which this rock was 

 formed.* 



* These porphyries contain pinite, a mineral which I have also 

 found in the porphyritic veins of Ben Gloe and the adjoining 

 mountains. The granite of the mainland in the vicinity of Sky con- 

 tains spodumene, a substance which has not hitherto been observed in 

 Scotland. 



