THE ISLE OF EIGG. \ (53 



invariably describes the pitchstones of Eigg as obsidian, 

 and, in regard to the porphyry of the Scuir, says, vol. ii. 

 p. 457, " Je la nommerai Obsidienne lithoide, pour la dis- 

 tinguer de 1' obsidienne vitreuse, dont elle seroit une sous- 

 espece." A confusion of pitchstones and obsidians is al- 

 ways unaccountable : they may be almost the same in ori- 

 gin and chemical compositions ; but they belong to two dif- 

 ferent series, are of distinct relative ages when viewed 

 generally, and have each well marked distinguishing cha- 

 racters. We now conclude this part of the subject by 

 merely stating, that, from the facts which we have men- 

 tioned in regard to the geology of Eigg, there appears 

 reason to draw the following inferences, which are almost 

 recapitulations of what we have already said, viz. that 

 there must have occurred, 1st, The horizontal disposition 

 of the oolitic strata. 2d, The protrusion of those rocks 

 which constitute the great trap-formation of the island. 

 3d, The eruption through these of the pitchstone masses. 

 4th, The invasion of one of these by a mass of felspar, to 

 which is probably due the shifts, to which, in several places, 

 they appear to have been subjected to. Plate V. fig. 3, ex- 

 hibits a section of Eigg, partly ideal, and is intended to 

 shew the probable relations of the several rocks. 



