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PROC'EEDTK'GS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



may be inferred from the fact that the pitchstone of the Sour, as 



will be shown in the sequel, is much 



younger than the rocks of the basaltic Fig, 6. Basalt Veins tra- 



versing interhedded Dole- 

 rites, Kildonan, Eigg. 





plateau. The pitchstone of most of the 

 veins differs, indeed, microscopically 

 from that of the Scur ; but the latter 

 varies greatly even within itself, so that, 

 though no evidence exists of any pitch- 

 stone vein having ever been connected 

 with the rock of the Scar, we may, pro- 

 visionally at least, class all the pitch- 

 stones together as the latest of the 

 igneous rocks of Eigg. 



Four separate veins of pitchstone 

 have been noticed in Eigg. The best- 

 known and most clearly exposed veins 

 are two which traverse the dolerite beds 

 at Budh an Tangairt, near the famous 

 Uamha Fhraing, or Frank's Cave, on 

 the south side of the island. The 

 eastern vein (fig. 7) consists, in its upper 

 part, not of pitchstone, but of a pale compact quartziferous porphyry 

 or felstone, like that of Scorr Sealleadh*. It is exceedingly hard, 

 spHntering under the hammer Yxg. 7. Vein of Pitchstone traversing 

 with a metalhc sound It Dolerite, Rudh an Tangairt. 



weathers with a yellowish or 

 reddish tint, which extends for 

 an inch or two into the stone, 

 and shows numerous cavities, 

 resulting apparently from the 

 decomposition of felspar crys- 

 tals. Towards the margin of 

 the vein it assumes a laminar 

 texture, in plates which are 

 in a general sense parallel to 

 the walls of the vein. Exa- 

 mined in thin section with 

 the microscope, this rock 

 shows a curious confused mass 

 of minute needle-like or hair- 

 like bodies, with opaque par- 

 tially decomposed grains of 

 pyrites, or possibly titaniferous 

 iron, and a still more decom- 

 posed brown mineral. The texture closely resembles that of some 

 of the pitchstones. The vein has a thickness of about 2| feet, and 



* An engraving of this vein is given by Jameson in his work already cited, 

 vol. ii. p. 45. The felspathic rock he terms " hornstone ;" it is called " chert " 

 by Macculloch, and " porphyritic compact felspar " by Hay Cunningham. It 

 differs in minute structure from evey other rock with which T am acquainted, 

 and, as stated in the text, more nearly resembles pitchstone. 



