154 MR CUNNINGHAM ON THE GEOGNOSY OF 



are also traversed by a vertical vein of compact basalt. 



Greenstone, 

 Basalt, and 



; Amygdaloid. 



\ 



__\. Vein of Basalt. 



X 



Pitchstone 

 Porphyry in 

 imperfect 



The western extremity of the porphyry range presents 

 phenomena similar to those afforded at the eastern : it 

 there, in a lofty coast section, sinks through the usual 

 trap-rocks, the several beds which occur on the one side 

 of the porphyry coinciding, in position and mineral cha- 

 racter, so strictly with those on the other, that their former 

 continuity can never be called in question. The pitch- 

 stone-porphyry of the Scuir alternates several times with 

 a porphyritic felspar. From the whole of their relations, 

 however, it is evident that they are of one and the same age. 

 Strictly speaking, the rock of Scuir Eigg cannot be 

 considered as a true pitchstone-porphyry, inasmuch as the 

 base of it, as Professor Jameson originally remarked, ap- 

 pears to be intermediate between basalt and true pitch- 

 stone : it may be better, however, still to view it as a 

 pitchstone-porphyry, than, by changing a received name, 

 endeavour to raise a mere variety into a distinct species. 

 To the eye, when unassisted by the lens, the base of this 

 rock appears perfectly homogeneous ; sometimes, however, 

 it is minutely granular, and of a black colour, with a slightly 

 dull, resinous lustre. The fracture is irregular and ap- 

 proaches to conchoidal. Before the blow-pipe, it fuses 

 into a semi-transparent greyish glass, and, when breathed 

 upon, emits an argillaceous odour. The porphyry-base 

 has a lower degree of hardness than true pitchstone, and 



