THE 



GEOLOGICAL MAGAZINE, 



No. XCI.— JANUARY, 1872. 



I. — On the Miceosoopic Structure of the Pitchstones of Arean. 



By S. Allport, F.G.S. 



(PLATE I.) 



THE Island of Arran has long been celebrated among geologists 

 for the variety and extensive develojDment of its igneous rocks. 

 The basaltic group is well represented in its various forms of struc- 

 ture and modes of occurrence ; the more highly silicated series by 

 felsites and porphyrites, forming dykes and huge amorphous or 

 columnar masses ; while the glassy varieties are illustrated by dykes 

 and veins of pitchstone. All these rocks may be readily studied in 

 the small area comprised within the southern half of the island, the 

 greater part of which consists of eruptive rocks, and sandstones of the 

 Carboniferous period. It is among these sandstones that most of 

 the igneous rocks have been intruded. Some, however, are evidently 

 interbedded and contemporaneous, forming great sheets of melaphyre 

 between the Carboniferous strata. 



In the northern half of the island the geological features are 

 entirely different. The whole of the central portion is occupied by 

 granite, rising here and there into mountain-masses with sharp 

 serrated peaks. The granitic district is roughly circular, with a 

 diameter of seven to eight miles, and is completely surrounded by 

 a band of Silurian slates ; the latter are bordered on the north, east, 

 and south by Old Eed Sandstone, or Lower Carboniferous rocks. 

 Trap dykes frequently occur here also, and there are dykes of pitch- 

 stone in the granite. 



A visit to the island in June last enabled me to collect a good 

 series of specimens of the intrusive rocks, including several very 

 interesting varieties of pitchstone. Of these I have prepared many 

 thin sections for microscopic examination ; and as few are aware of 

 the singular beauty of their structure, I propose to describe the most 

 characteristic varieties in the present communication. 



In all the specimens examined, the mass of the rock is found to 

 consist of a structureless glassy base, in which may be seen a variety 

 of crystalline minerals, some of microscopic size, others much larger, 



VOL. IX. — NO. xci. 1 



