G. W. Tyrrell — Petrology of Arran. 305 



YI. — The Petrology of Aekan". 



By G. W. Tyrrell, A.E.C.Sc, F.G.S., Assistant to the Professor of Geology, 

 University of Glasgow. 



1 . The Riebeckixe-orthophyke of the Holy Isle. — Four occur- 

 rences of acid and sub-acid igneous rocks containing the rare 

 soda-amphibole riebeckite have been recognized in Great Britain, 

 of which that of Ailsa Craig is the best known. The occurrences 

 of Ailsa Craig ^ and Mynydd Mawr, Carnarvonshire,^ are riebeckite- 

 microgranites or paisanites. They contain ragged moss-like areas 

 of riebeckite, together with microphenocrysts of quartz and alkali- 

 felspar, in a raicrocrystalline groundmass of quartz and felspar. 

 Riebeckite was also found by Dr. Teall in the granophyre of Meall 

 Dearg and the neighbouring area of Druim an Eidhne, Skye.^ 

 Harker described the riebeckite in these rocks as occurring in two 

 forms, one having the usual ragged, sponge-like appearance, and 

 the other being idiomorphic, the faces in the prism zone being 

 well-defined, but with irregular terminations.* The fourth occurrence 

 differs somewhat from the others. This rock occurs as an intrusion 

 into the Upper Old Red Sandstone, or Calciferous Sandstone of 

 Easter Eildon Hill, Melrose, and was described by Barron as 

 riebeckite-trachyte or phonolite.^ It consists principally of sanidine, 

 occurring both as microphenocrysts and in the groundmass, with 

 interstitial patches of riebeckite, and a little nepheline. Harker 

 describes the rock as an orthophyre.^ 



It is interesting to be able to add a fifth occurrence of a riebeckite 

 rock in the so-called felsite of the Holy Isle, near Lamlash, Arran. 

 It is noteworthy that Ailsa Craig is only 18 miles from this locality. 

 The Holy Isle forms a steep and rugged conical hill rising to a height 

 of 1,030 feet, and fitting, so to speak, at a distance of 1^ miles, 

 into the recess of Lamlash Bay. It is a prominent landmark in 

 the Firth of Clyde, as prominent almost as Ailsa Craig itself, and 

 moreover of much the same shape. The island is extended in 

 a N.N."W. to S.S.E. direction for 2 miles, and has an average 

 width of half a mile. According to the Survey memoir and map 

 of this district the Holy Isle consists of thick sills of felsite 

 intruded into the third group (red sandstones and conglomerates) 

 of the Lower Trias.'' The latter occupies a narrow strip on the 

 western and southern sides of the island, and is also intruded by 

 a dolerite mass which forms a continuation of the Kingscross 

 sill and by basalt dykes. The felsites, however, are probably later 

 than the dolerites and basalts. According to the memoir they belong 

 to the group of felsite sills, which seldom show free quartz and 

 are usually non-porphyritic. These are believed to be later than 



1 Teall, Min. Mag., ix, 219-21, 1891. 

 - Harker, Bala Vole. Ser. Carnarvon, 1889, pp. 50-2. 

 •' Q.J.G.S., 1, 219, 1894. 



* Tertiary Igneous Bocks of Skye (Mem. Geol. Surv.), 1904, p. 158. 

 ^ Geol. Mag., 1896, p. 376. 



^ Petrology for Students, 4th ed., 1908, p. 129 ; see fig. 33b, p. 128. 

 '' The Geology of North Arran, South Bute, and the Cumbraes (Mem. Geol. 

 Surv.), 1903, p. 70. 



DECADE v. — VOL. X. — NO. Vn. 20 



