Vol. 59.] THE THIEE AND IONA MARBLES. 103 



(d) The present foliation of the marble and possibly in part of 

 the gneiss, together with the various dynamic' phenomena 

 displayed by the marble (and gneiss), is the result of sub- 

 sequent djmamic metamorphism unaccompanied by any marked 

 thermal metamorphism. 



lona. 



The marbles of lona were very probably originally similar to 

 those of Tiree, but have, in most cases, suffered more severely from 

 dynamic melamorphism. 



EXPLANATION OF PLATES VI & VII. 



Plate VI. 



Fig. 1. Incipient cataclastie structure in carbonates. Fields south-east of 

 Baiephetrish Hill. X 33, crossed nicols. (Seep. 98.) Slide 1027, 

 Author's collection. 

 2. Twinned and bent crystal of sahlite, in the mylonized limestone of the 

 crush-conglomerate, Port Abhuinn. X 20, crossed nicols. (See 

 p. 100.) Slide 1033, Author's collection. 



Plate VII. 



Fig. 1. White limestone with forsterite (and sahlite) ; the lines of crushing are 

 well seen in the carbonate-matrix. From small quarry in field south- 

 east of Baiephetrish Hill. X about 9. (See pp. 98 & 100.) Slide 1022, 

 Author's collection. 



2. Cataclastic structure in pink marble ; bending, shearing, and break-up 



of calcite-grains. The large transparent grain is orthoclase. Port- 

 Abhuinn marble, x 12. (See pp. 94 & 100.) Slide 1034, Author's 

 collection. 



3. Aggregate composed of coccolite and scapolite with sphene, (blue) 



apatite, calcite, and mica. Natural Arch, shore, Baiephetrish. x 15. 

 (See p. 96.) Slide 1040, Author's collection. 



4. Silicate-aggregate, with abundant calcite and dolomite ; the carbonates 



uncrushed. Same locality, x about 13. (Seep. 99.) Slide 102;"), 

 Author's collection. The nearly central small dark patch is an 

 appearance due to local incipient breakdown of a calcite-grain. 



5. Sahlite in the same slide as fig. 1 ; basal cleavage well developed. 



X about 8. 



6. Idiomorphicamphibole, in fine-grained pink marble, Baiephetrish Quarry. 



X about 11. (See p. 100.) Slide 1019, Authors collection. 



Discussion. 



The Peesident spoke of the fine series of microscope-sections 

 exhibited by the Author, and of the evidences of movement and of 

 mylonization which they presented. The proofs of contact-alteration 

 of the calcareous rock by the gneiss seemed clear, and the association 

 of minerals was reminiscent of that shown in the case of the 

 Ledbeg Marble. Was it impossible that the Tiree lenticles, which 

 had the same trend as the Ledbeg rock, were relics of a south- 

 westerly extension of the same calcareous formation? The con- 

 clusion that the Tiree Marble was a limestone which had been 



