306 G. W. Tyrrell — Petrology of Arran. 



the contrasted group of quartz-porphyries, since they intrude the 

 latter and are seldom pierced, as the quartz-porphyries are, hy 

 the ordinary basalt dykes. ^ In several places the ' felsite ' of the 

 Holy Isle forms great vertical cliffs showing a rude columnar 

 jointing. On the western side of the island the igneous rock clearly 

 rests on horizontal red sandstones and has a sill-like appearance. 



The rock composing most of the island is a compact, hard, greyish, 

 ' felsitic ' material, which weathers deeph' with a yellowish crust. 

 On breaking ojDen one of the large, loose, angular blocks which strew 

 the slopes, a very regular concentric distribution of colour, due to 

 gradations of alteration, is seen. The exterior yellow zone of 

 decomposed material is often an inch thick, and, with regular 

 gradations of colour, passes into fresher and fresher rock, and 

 ultimately into a kernel of quite fresh rock. The latter is very 

 compact, dark grej* in colour, and shows minute, flashing cleavage- 

 plates of sanidine. The rock frequentlj' breaks into angular 

 tabular fragments conditioned by a close horizontal jointing, and 

 on steep slopes illustrates the formation of screes on a small scale to 

 perfection. 



Microscopically the rock consists of microporphyritic sanidine in 

 a groundmass of smaller lathy crystals of the same mineral, with 

 riebeckite and minute specks of iron-ore. The sanidine occurs in 

 euhedral, rectangular, simply-twinned crystals, which are evenly and 

 numerously scattered all over the field. The groundmass consists 

 of subhedral laths of sanidine, closely packed together, with abundant 

 riebeckite. The latter appears to occur in two forms, as in the 

 Beinn Dearg granophyre. One set is in small independent prisms, 

 parallel-sided in the prism zone, but with ragged irregular 

 terminations ; the other occurs in the well-known ophitic, mossy, 

 or sponge-like masses. The latter are generally altered to an 

 indeterminate yellow mineral, but the prismatic crystals are quite 

 fresh and show the usual pleochroism from indigo-blue, through 

 greenish-blue, to yellowish-green. Search was made for nepheline, 

 but microscopic and staining methods failed to reveal any trace of 

 this mineral. 



The small laths of the groundmass show some approach to flow- 

 orientation, but the numerous, short, stumpy, rectangular prisms 

 of sanidine, dispersed in all directions throughout the rock, determine 

 the texture as orthophyric. This, together with the intrusive mode 

 of occurrence, renders the term riebeckite-orthopliyre more appropriate 

 than rieheckite-trachyte. 



The rock differs from that of Ailsa Craig in being entirely devoid 

 of quartz, which occurs both in the groundmass and as micro- 

 phenocrysts in the latter rock. It is curious that these two islands, 

 so similar in general appearance, should both be composed of 

 riebeckite-bearing rock. If it were possible to examine the contacts 

 of the Ailsa Craig mass, it is not improbable that it would be found 

 to have geological relations similar to those of the riebeckite- 

 orthopliyre of the Holy Isle. The closest petrological affinities of 

 the Holy Isle rock, however, are with the riebeckite-phonolite or 

 ^ Geology of North Arran, etc., p. 91. 



