Vol. 59.] THE TIBEE AND ION A MARBLES. 101 



aggregates occurring in the limestone near the ' Natural Arches ' on 

 the shore. The grains are often elongated in the direction of the 

 vertical axis, but are not idiomorphic ; in thin sections they are 

 practically colourless. The grains are only faintly pleochroic, 

 showing a bright sky-blue for rays vibrating perpendicular to the 

 vertical axis, and a paler greenish-blue for rays vibrating parallel 

 thereto. Individual grains vary in colour from bright sky-blue to a 

 duller bluish-green. Possibly this apatite is the mineral spoken of 

 by Heddle l as a ' pale-blue watery variety ' of sahlite. Mr. W. C. 

 Hancock, B.A., has carefully analysed the blue apatite, with the 

 following result: — 



CI =1-85: SiO., = r5; CaO = 53-92; P.,0, = 39-55; and H 2 (hygroscopic) = 3-16. 

 Total = 99-98. Specific gravity = 3-20. 



This analysis shows that the blue apatite of Tiree is one of the 

 comparatively rare chlor-apatites, and much resembles analyses of 

 chlor-apatite given by Vcelcker.- 



S pin el. — A few grains of bluish- green spinel (not hitherto 

 recorded from Tiree) are scattered very sparingly in a variety of 

 marble (No. 3, p. 94) exposed in the field south-east of Balephetrish 

 Hill. The accompanying silicates are: — abundant forsterite, and 

 scarce mica, tremolite, and sahlite. The spinel is quite isotropic; 

 and in thin chips nearly colourless. Mr. G. T. Prior, M.A., F.G.S., 

 has kindly determined the specific gravity as 3*635. The spinel is 

 probably referable to pleonaste. 



VI. IONA. 



Marble is exposed on the shore south of Hun Dugaidh ; a 

 long ' slack ' leads down to the — from the sea — rather inaccessible 

 beach known as the ' Marble Quarry.' Much white marble occurs 

 here : portions are greenish or black. It has a silky, almost- 

 schistose aspect ; it seems to occur as a band or lenticlc in the 

 gneissose rocks. The colourless varieties consist of calcite, with 

 some serpentine and tremolite, the last-named in small needles often 

 felted together. In the dark patches serpentine is more abundant, 

 and may practically compose the rock. The rocks immediately 

 associated with the marble include actinolite-felspar schists with 

 epidote and pyrite (north wall of 'slack') ; hard white rocks, ex- 

 cessively fine-grained, resembling the marble but composed of felspar 

 and muscovite (?) with zoisite. Associated with these was a band 

 of mica-schist-like rock, composed of finely granular quartz, mica, 

 and felspar, with actinolite and pyrite, and exhibiting well-marked 

 foliation. This rock suggests an altered sediment. There are, 

 moreover, bands and streaks of dark rock, hard and heavy, in the 

 marble itself; these are actinolite-felspar-schists, with much pyrite 

 and some epidote. The orientation of the needles of actinolite is 

 very marked, the ether-axis c being parallel to the foliation. All 

 these rocks appear to have been entirely reconstructed. 



1 Trans. Roy. Soc. Eclin. vol. xxviii (1879) p. 459. 



2 Rep. Brit. Assoc. 1857 (Dublin) Travis. Sect. p. 59. 



