100 MR. A. K. COOMARASWAMY ON [Feb. I903, 



that the sahlite may be dark-green, light-green, or grey, and does not 

 mention coccolite as actually occurring in the limestone at all. A 

 more detailed chemical examination of these pyroxenes with reference 

 to colour seems desirable. 



The pyroxenes are often twinned, and even distorted, in those 

 varieties of limestone which have suffered most severely from the 

 pressures (PI. VI, fig. 2). 



Am phi bole. — Dark-green amphiboles are very characteristic of 

 the pink Balephetrish marble, and are often of large size, and then 

 sometimes intergrown with carbonates. Individuals are frequently 

 idiomorphic (PJ. VII, fig. 6). The plcochroism is a pale greenish- 

 straw, b olive-green, c bluish-green ; the extinction-angle on flakes = 

 about 23°. Colourless tremolite occurs in other varieties of limestone. 



Forsterite. — This mineral (not hitherto recorded from Tiree) 

 occurs abundantly in some varieties of limestone, especially in the 

 white limestone exposed in a field south-east of Balephetrish Hill. 

 Crystals are never idiomorphic, but elongated individuals extinguish 

 in the direction of their length, and examination in convergent 

 polarized light shows that the axial plane is at right angles to the 

 direction of elongation. Partial or complete serpentinization, com- 

 mencing along the irregular cracks, is typically exhibited. 



The forsterite from a specimen (tig. 3, p. 94 & PI. VII, fig. 1) 

 of marble from the south side of the field has been analysed by 

 Mr. W. C. Hancock, B.A. Weathered, but carefully selected crystals 

 were used for this analysis, which confirmed the identification of 

 forsterite. The specific gravity was 2 -SO. 



Scapolite. — This mineral was found by Ileddle in the residues 

 from the pink Balephetrish marble, in rare minute crystals, with 

 the forms «(100), wi(HO), r(lll), associated with sahlite and white 

 biotite. His analysis is as follows ] : — 



SiO.,=48023; Al 2 O 3 =22'098; Fe a 8 =3-159; FeO= 1-508; MiiO=0o38; 

 CaO=7753; MgO=2769; K 2 O=6058; IS T a,O=1070 ; HX>=5b94. 



I have found it much more abundant, in larger, not idiomorphic 

 grains in mineral-aggregates (associated with coccolite, sphene, and 

 apatite), in modified gneiss-inclusions, and in contact-zones. 



Sphene. — Small, lustrous, blunt-angled crystals, of a watery- 

 brown colour, occur sparingly in the pink marbles. Larger grains 

 occur more abundantly in the coccolite-scapolite-sphene aggregates, 

 and in some varieties of contact-rock or modified gneiss-inclusions. 

 The grains have sometimes quite a pinkish colour ; they were 

 mistaken by Jameson ~ for garnet. 



Mica. — Bather decomposed, colourless flakes of mica occur in 

 some varieties of limestone and in mineral-aggregates. No typical 

 phlogopite occurs. Dark-brown mica is occasionally seen in mineral- 

 aggregates. 



Apatite. — Grains of sky-blue apatite (not hitherto recorded from 

 Tiree) are found rather plentifully in the coccolite-scapolite-sphene 



1 ' Mineralogy of Scotland ' vol. ii (1901) p. 53. 



2 ' Mineralogy of the Scottish Isles' vol. ii (1800) p. 32. 



