104 THE TIREE AND IONA MARBLES. [Feb. I903. 



first intruded upon, enveloped, and metamorphosed by igneous rock, 

 and that afterwards 'the resulting complex had been subjected to 

 dynamic movements bringing about mylonization, was simple, and 

 appeared to agree with most of the phenomena cited. It would be 

 interesting, however, to know whether there was anything apparent 

 to disprove the theory that the injection and the main dynamic 

 action were simultaneous ; or, in view of the experiments of Adams 

 & Nicolson, that the crystals of calcite, etc. might not have 

 developed and enlarged even under pressure ; or that the original 

 limestone might not have undergone dynamic action before the 

 injection ; or that the entire complex itself might not have been 

 squeezed and yielded more than once afterwards. 



Prof. Bonne r said that his knowledge of the Tiree Marble was 

 founded on hand- specimens and rock-slices only, as he had never 

 visited the place. Still, in the Alps, he had had so many oppor- 

 tunities of studying the effects of pressure on crystalline limestones, 

 that he could venture to say that the suggestion thrown out by the 

 President as to the coarser calcite-grains in the marble being 

 recrystallized, was no more probable than that the felspar-grains in 

 the ' mylonites ' of Glen Laggan were recrystallized. The coarse 

 calcite might be sometimes seen to have been protected between two 

 grains of sahlite, like a bay between headlands. He called attention 

 to some analogies between the Tiree rock and the well-known 

 crystalline limestones of Cote St. Pierre (Canada), and said that he 

 did not think any close connexion existed between it and the 

 Ledbeg Marble. The paper to which the Society had listened 

 appeared to him to make a valuable addition to knowledge. 



Prof. Sollas thought that the Ledbeg limestone owed its character 

 to contact-metamorphism, that of Tiree to this action and something 

 more ; for it appeared to be a very mixed product, and some of its 

 minerals might have been originally deposited along with calcareous 

 sediment, just as olivine-sands are now being mingled with calca- 

 reous sands on the shores of Sawaii. Some of the felspar of the 

 Tiree Marble presented evidence of secondary growth ; and that 

 felspar might be formed by subsequent growth within a 'calciphyre' 

 was shown by Issel's observations on radiolaria included within 

 albite-crystals, which occurred as constituents of the so-called 

 ' calciphyre ' of Rovegna in the Val di Trebbia. 



The Author said that he saw no reason for connecting the 

 Ledbeg and Tiree Marbles, the former being post-Torridonian, 

 the latter in all probability pre-Torridonian. It was quite clear 

 that recrystallization had not taken place since the production of 

 cataclastic phenomena. Contact-phenomena were unmistakable in 

 places. He thought that there was little to suggest a detrital 

 origin for the accessory minerals. With the exception of the, not 

 very abundant, felspar, it would be very hard to imagine a volcanic 

 origin for these. 



