102 ME. A. K. COOMAKASWAMY ON [Feb. I9O3, 



A small exposure of highly sheared silky limestone occurs near 

 the commencement of the path at the beginning of the grassy ilat 

 (raised beach) above St. Columba's Bay (Port-a-Churaieh) ; it forms 

 a band in the gneiss, with corresponding foliation. It contained a 

 few small silicate-augen : one consisting of colourless monoclinic 

 pyroxene, with secondary tremolite and calcite ; another of felspar 

 and calcite, with zoisite and amphibole. The felspar is mostly 

 triclinic ; there is some orthoclase and orthoclase-microperthite ; in 

 the latter the inclusions of triclinic felspar have sometimes given 

 rise to secondary calcite, producing a (secondary) banded intergrowth 

 of calcite and orthoclase. 



Finally, there is a band of limestone or ophicalcite (locally called 

 sun stone) in the gneiss about a quarter of a mile north-east of 

 Port Ban, on the wes'tern coast of Iona. The rock is less crushed 

 than those above described, and consists of granulated calcite with 

 equally abundant flakes of colourless mica (often bent) and rounded 

 grains of serpentine, the last-named very possibly a pseudomorph 

 after forsterite, and also grains of nearly colourless sphene. A single 

 large crystal of decomposed pyroxene was also observed. 



The limestones just described from lona are included in the 

 gneiss. The latter is of varied character, though hornblendic types 

 are perhaps most usual ; it has nearly everywhere suffered much 

 from pressure. 



(Sedimentary rocks suggestive of Torridon Sandstone occur along 

 the eastern shore of Iona, especially in the northern half of the 

 island ; they include felspathic grits and finer slaty or shaly bands. 

 They become very much sheared, especially as the junction with 

 gneiss is approached ; there would be much difficulty in drawing a 

 satisfactory boundary. The limestones above described are not 

 associated with these sediments, but entirely included in the gneisses, 

 and seem to correspond to rocks such as the Tiree Marble, but 

 which have suffered much more from the effects of pressure, so that 

 we cannot study the original relations or even point to original 

 minerals. The ophicalcite from near Port Ban, however, more 

 nearly resembles the Tiree Marble ; differing chiefly in the absence 

 of any portion of the mineral now replaced by serpentine. 



VII. Summary and Conclusions. 



Tiree. 



(a) The various patches of marble occur as lenticular masses in 



the gneiss. There is no evidence as to the origin of the 

 limestone. 



(b) The development of the accessory minerals and probably also 



the original more coarsely crystalline character of the marble, 

 are the result of contact-metamorphism at high temperature. 



(c) The hornblendic and acid orthogneisses associated with the 



marbles were the agents in this thermometamorphic process. 



