Vol. 59.] THE TIREE AND IONA MARBLES. 97 



minerals are completely interlocked, and there is no actual line of 

 junction seen under the microscope, although an abrupt change is 

 evident. The rock has suffered from frequent minute faulting and 

 slight shearing, the effects of which are quite distinct in the gneiss 

 and contact-zone, but are lost as they pass into the compact marble. 

 The original relations are not obscured. It is rather curious that 

 the only rock here seen in actual contact with the marble should be 

 this quartzo-felspathic gneiss or granulite, while the prevailing rocks 

 at Balephetrish are of a dark hornblende-plagioclase type. 



At many points on the shore contacts of marble and gneiss 

 (usually of acid type) are to be seen. In some cases, as, for example, 

 near the ' Natural Arches,' a zone of grey rock composed of mono- 

 clinic pyroxene separates typical limestone from gneiss ; in other 

 cases, no very striking contact-phenomena attract attention. 



Enclosures of gneiss in marble appear often to have been much 

 modified by the absorption of lime, with the development of pyroxene, 

 scapolite, and sphene; in one case (Port-Abhuinn marble) a large in- 

 clusion of gneiss was separated from the typical marble by a narrow 

 zone of pyroxene-scapolite-sphene-rock, with apatite and calcite, 

 essentially resembling the rock of a narrow contact-zone described 

 above, and of the mineral-aggregates occurring in the marble at 

 the ' Natural Arches.' 



The presence of accessory minerals in the limestone is in itself 

 to be regarded as evidence of contact-metamorphism. It is shown 

 below that the rounded character of some of these affords no 

 evidence of detrital origin, such as has been suggested l ; grains 

 of rounded coccolite and sphene are associated with idiomorphic 

 hornblende. I have no doubt that all these minerals have crystal- 

 lized in situ : probably under conditions analogous to those of 

 minerals crystallizing iii a cooling magma, when, moreover, the 

 limestone perhaps existed in a state akin to fusion. 



IV. Dynamic Phenomena. 



In some varieties of limestone foliation is inconspicuous, and the 

 silicates are frequently idiomorphic. In others it is very distinct; 

 and the included minerals are rounded, and form little augen round 

 which it sweeps (fig. 4, p. 95). The rounded outline of the silicate- 

 minerals is frequently, however, an original character : thus the 

 grains of coccolite and sphene in the pink Balephetrish marble have 

 smooth rounded forms, but are associated with sharply idiomorphic 

 hornblende, showing that the rounded form is for these minerals a 

 characteristic habit rather than a secondary phenomenon. 



The effects of pressure are most conspicuous in the carbonates. 

 These are usually present as the fine-grained, granular, compact 

 matrix, in which various silicates are embedded. The various thin 

 sections examined show most clearly that the minutely granulated 

 condition is a result of the complete breakdown of larger grains, 



1 G. A. J. Cole & W. J. Sollas, Sci. Proc. Roy. Dublin Soc. vol. vii (1891) 

 p. 124; A. Harker, ' Petrology for Students ' 2nd ed. (1897) p. 317. 



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