Note 3 — Chapter IV · ST. HELENA

This circumstance has been observed (Lyell, “Principles of Geology,” vol. iv, chap. x, p. 9) in the dikes of the Atrio del Cavallo, but apparently it is not of very common occurrence. Sir G. Mackenzie, however, states (p. 372, “Travels in Iceland”) that all the veins in Iceland have a “black vitreous coating on their sides.” Captain Carmichael, speaking of the dikes in Tristan d’Acunha, a volcanic island in the Southern Atlantic, says (“Linnæan Transactions,” vol. xii, p. 485) that their sides, “where they come in contact with the rocks, are invariably in a semi-vitrified state.”