THE ISLE OF EIGG. 155 



affords to the knife a greyish-white streak. Small veins 

 of magnetic iron-ore occur imbedded in this rock. The 

 crystals, which produce the porphyritic structure, consist en- 

 tirely of a brownish or yellowish- white, and highly lustrous 

 glassy felspar, varying in abundance in different places ; 

 they fuse under the blow-pipe into a transparent glass. 

 The structure of the pitchstone-porphyry, on the great 

 scale, is most regularly columnar. The position of the 

 columns which form the highest parts of the mountain ap- 

 proaches to verticality ; nearer the base, however, they 

 become inclined, and afford numerous beautiful examples 

 of bent groups. At the eastern extremity of the Scuir, 

 a bed of inclined pillars is included between pillars hav- 

 ing a perpendicular arrangement. This is very striking, 

 and may be seen though the observer be at a considerable 

 distance from the base of the hill. In the number of 

 their sides much variety may be observed in the columns ; 

 and amongst the debris which is accumulated at the foot 

 of the range, it is easy to discover differently-shaped 

 pentagons, hexagons, eptagons, and octagons, with others 

 of irregular, quadrangular, and triangular forms. In 

 regard to these columnar concretions, there are two cha- 

 racters, which, though very generally evinced by green- 

 stone and basalt, are never to be detected in the pitch- 

 stone-porphyry, viz. in no instance do the pillars afford 

 that internal structure which is known by the name of 

 " ball and socket,"" or appear arranged in globular dis- 

 tinct concretions :* associated with the rock of the Scuir, 



* Necker de Saussure, when speaking of the rock of the Scuir, states, 

 that Professor Jameson lias, in his opinion, improperly styled it a por- 

 phyry with a base of pitchstone. This is an error, for the Professor, in 

 his Mineralogical Travels, page 47, vol. ii., affirms, that the porphyry has 

 " a basis intermediate between basalt and pitchstone," a description 

 which most exactly describes its aspect. 



