158 GEOLOGY OF ARRAN. 



intermobility, under the influence of electrical and other 

 polar forces. The "pyroxenic mineral," of which the acicular 

 prisms consist, is afterwards shewn to be augite, and not 

 hornblende an interesting confirmation of the view that this 

 mineral is produced when rocks cool rapidly from fusion, and 

 become glassy, while hornblende is the result of slow cooling, 

 when a stony substance is formed, as has been, already 

 remarked. These papers contain also many interesting par- 

 ticulars regarding the internal structure of the felspar and 

 quartz included in the pitchstones and porphyries; but we 

 cannot follow the subject farther. 



In alluding to our account of the hornstones associated 

 with the pitchstones and claystones in Arran, Mr. All- 

 port expresses the opinion that hornstone is simply an 

 altered pitchstone; indicating no doubt thereby an altera- 

 tion of incipient decomposition, and not a metamorphic 

 action as by a trap dike. Hornstoue is rather a meta- 

 morphic claystone, just as quartzite is a metamorphic sand- 

 stone, and it is thus a link or transition state between pitch- 

 stone and claystone. But, in fact, the three rocks occur 

 together, both here and at Tormore, under conditions which 

 shew that they were of cotemporaueous origin, and that 

 different rates of cooling or exposure to different agencies 

 of decomposition have produced the different external char- 

 acters. This is borne out by the microscopic observations 

 of Herr Zirkel and Mr. Allport. A specimen of " hornstone" 

 exhibited " under the microscope the characteristic structure 

 of the unaltered rock. The quartz crystals [imbedded] are 

 the same in every respect, but the felspar is no longer trans- 

 parent, and the clear green acicular crystals have lost their 

 colour, and appear as yellowish-brown specks." 



