98 ME. A. K. COOMARABWAMY ON [Feb. 1903 



fragments of which are sometimes preserved (see PI. VII, fig. 4), and 

 that the limestone as a whole must once have possessed a more 

 coarsely crystalline character, as was pointed out by Prof. Bonney 

 several years ago. 1 These phenomena are illustrated in PI. VI, fig. 1, 

 and PI. VII, figs. 1 & 2. Other examples of marble showing 

 cataclastic structure are described and illustrated by Messrs. Adams & 

 Nicolson, 2 who have reproduced experimentally the same phenomena. 

 If we accept the results of their experiments, we must conclude that 

 the deformation of the Tiree Marble is the result of pressure, acting 

 at a temperature not above 300° C. We are, then, observing the 

 effects of d3 T namometamorphism, free from complications due to 

 marked accompanying thermal metamorphism. This must apply also 

 to the gneisses with which the limestone is associated : a detailed 

 examination of these has not been made; they have certainly suffered 

 from minute faulting and local shearing, but to a small extent, com- 

 pared with the gneissose rocks of Iona, or, for example, of the Loch- 

 Maree district. The gneisses are of a general Lewisian type, and 

 include dark hornblendic rocks composing Balephetrish Hill, and also 

 quartzo-felspathic gneisses which are more abundant on the shore. 

 Coarse rocks of granuloid or pegmatitic aspect, and with incon- 

 spicuous foliation, occur near the road farther east. 



An interesting crush-breccia (fig. 5, p. 99) occurs at the junction 

 of limestone and gneiss, at Port Abhuinn, on the shore west of 

 Balephetrish. Here we find a foot or two of completely mylonized 

 limestone, having a smooth flinty conchoidal fracture, containing 

 numerous blocks of gneiss, which have also suffered from the 

 pressures. The calcareous matrix has a delicate ' flow-structure/ 

 resembling that of a rhyolite. Something of the same kind has 

 taken place quite locally near the spot marked ' Natural Arch ' on 

 the shore. There are gneissose streaks and patches in the lime- 

 stone, and the two rocks are sheared together. The marble is a fine- 

 grained calcite-rock, with diopside-grains more or less twinned and 

 strained and bent. The sheared gneiss consists of a fine-grained 

 foliated matrix of felspar and calcite-dust, with rounded scapolite and 

 triclinic felspars with undulose extinction, broken porphyritic augite, 

 and occasional hornblende and sphene. A dark intervening zone 

 contains augen of felspar, hornblende, and sphene, and is perhaps the 

 remnant of a contact-zone. It is more usual, however, for gneiss- 

 inclusions and mineral-aggregates (PL VII, figs. 3 & 4) in the 

 limestone to have been more or less protected from the extreme 

 effects of pressure. 



V. Notes on the Minerals. 

 The following minerals occur in the limestones or in the mineral- 

 aggregates found in them : — Calcite, dolomite, pyroxene, amphibole, 

 forsterite, scapolite, mica, sphene, apatite, orthoclase, spinel, and 

 serpentine. 



1 'The Effects of Pressure on Crystalline Limestones' Geol. Mag. 1889, p. 483. 



2 ' An Experimental Investigation into the Flow of Marble ' Phil. Trans. 

 Boy. Soc\ vol. cxcv (1901) a p. 387 & pis. xxii-xxv. Compare especially pi. xxv, 

 figs. 3 & 4 with my figures. 



