536 S. AUjJort — On the Igneous Rocks of Arran. 



IV, — On the Mickoscopic Stktjctuke of the Pitohstones and 

 Felsites of Akran. 



By S. 'Allport, F.G. S. 



IN my last communication^ I gave some account of all the varieties 

 of Arran pitclistones then known to me, and referring to other 

 closely allied rocks, intimated an intention of describing them on 

 another occasion. 



A second visit to the island during the past summer has not only 

 enabled me to collect the requisite materials, but has also supplied 

 me with a few additional varieties of pitchstone ; some remarkable 

 for their microscopic structure, others of considerable interest to the 

 petrologist, as they exhibit a transition from the true glassy texture 

 to that of the dull compact "hornstones" and felsites. 



Intimately associated with the pitchstones, there frequently occur 

 dykes and other intrusive masses of light-grey rocks of a felsitic or 

 trachytic character ; they are all quartziferous, and present several 

 well marked varieties. As these rocks have not been previously de- 

 scribed, I purpose giving some account of their structure and mode 

 of occurrence, but will, in the first place, complete my description of 

 the pitchstones. 



In my former paper I described the pitchstones on the east and 

 west coasts as occurring in intrusive veins and dykes, and subse- 

 quent examination has not only confirmed this view, but has also 

 shown that the sandstones are greatly altered along the line of 

 junction. Prof. Zirkel, on the other hand, states that the vein on 

 the Corriegills shore has not produced the slightest change on the 

 sandstones, and that it is regularly interbedded with them.- It 

 became necessary, therefore, to make another careful examination of 

 the locality. The result was, that the sandstone in contact with, or 

 in close proximity to, the pitchstone, is greatly indurated in places, 

 while in others it is comparatively soft ; but in these instances there 

 is invariably an escape of water at the junction of the two rocks, 

 and a partial disintegration has been the result; an examination, 

 therefore, of such places only, might easily mislead an observer. 

 As regards the relative positions of the two rocks, I have nothing 

 to add to the previous account ; in all the localities examined the 

 pitchstone is clearly intrusive. 



In order to insure a tolerably complete investigation of the rocks 

 described in this and the former paper, specimens exhibiting any 

 variations in structure or appearance were taken from each locality ; 

 and of these I have prepared forty-one thin sections for microscopic 

 examination. 



1 See Geol. Mag. Vol. VIII. pp. 448-450. 



2 Zeits. d. d. geol. Ges., xxiii., p. 1. 



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