347 



black cross in polarized light but consist of crystalline patches which 

 give uniform extinction over considerable areas. These patches are, 

 however, longer in the radial than in the tangential direction. Where 

 the spherulites are closely set they mutually interfere, and divide up 

 the mass into a polygonal net-work. The sutures appear as clear lines 



99-91 99-81 



99-3 



99-73 99-90 100-80 



Sp. Gr. 



2-340 



I. "Claystone," Arran. Analysis by M. M. TAIT. Quoted from Bryce's Geology of 

 Arran, p. 203. 



II. " Pitchstone," Arran. Analysis by same observer and quoted from the same source. 



III. " Pitchstone," Arran. Analysis by J. H. PLAYEB. (Previously unpublished). 



IV. "Hornstone" nodule in Pitchstone from vein occurring near King's Cove. Analysis 

 by J. A. PHILLIPS. G.M., 1872, p. 540. 



V. Red felsite or " hornstone " from same locality. Analysis by J. A. PHILLIPS. G.M., 

 1872, p. 540. 



VI. Spherulitic or globular felsite. Corriegills shore, Arran. Analysis by J. A. PHILLIPS. 

 G.M., 1872, p. 540. See Fig. 1, PI. XXXIX. 



Felsitic lavas and tuffs are extensively developed in the Island of 

 Mull. These have not, however, as yet been described in detail. The 

 Scuir of Eigg is composed of a porphyritic pitchstone (see Fig. 5, 

 Plate XXXIV.) in which crystals of quartz, sanidine, and augite occur 

 It shows a well-marked peiiitic structure, and in this respect differs from 

 most of the Arran pitchstone. Arborescent microlites are absent. Rhyo- 

 lites and felsites occur in the Shetland Isles, associated with Old -Red 

 Sandstone rocks. These have been described and figured by Messrs. 

 PEACH and HORNE. (1) 



Ireland. The felsitic rocks of Ireland, like those of Scotland, 

 have not received much attention at the hands of modern petro- 

 graphers. One of the most interesting rocks is the tridymite-bearing 

 liparite of Tardree,( 2 > Co. Antrim. 



Macroscopically this rock consists of porphyritic crystals of glassy 

 sanidine, plagioclase and smoky quartz in a pale yellow or white 

 trachytic or felsitic matrix. The hollow spaces contain small hexagonal 



(1) The Old Red Volcanic Rocks of Shetland. Trans. Roy. Soc., Edin. Vol. XXXII., 

 Part II., p. 359. 



LASAULX, T.M.M., Neue Folge. Band I. (1878), p. 410. 



