[Footnote 98: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 487.] It may be perhaps legitimate to infer that, while the fragments that fell back into the volcanic funnel, or which were detached from the sides of the vent, after having been exposed for some time to intense heat under considerable pressure, would suffer more or less metamorphism, those, on the other hand, which were discharged by the æriform explosions from the cool upper crust, on the first outburst of a vent, would not exhibit any trace of such a change. Where, therefore, we meet with a neck full of fragments of unaltered stratified rocks, we may suppose it to have been that of a short-lived volcano; where, on the other hand, the fragments are few and much altered, they may mark the site of a vent which continued longer active. The metamorphism of the fragmentary contents of a volcanic funnel by the action of ascending vapours has already been described in the case of one of the vents of the Carboniferous plateaux (vol. i. p. 404). One of the most curious and puzzling features in the contents of the tuff necks of the Fife coast is the occurrence there of crystals and fragments of minerals, often of considerable size, which do not bear evidence of having-been formed _in situ_, but have undoubtedly been ejected with the other detritus. Dr. Heddle has noticed the fact, and has described some of the minerals which occur in this way. The following list comprises the species which he and I have found:-- Hornblende, in rounded fragments of a glassy black cleavable variety. Augite, sometimes in small crystals, elsewhere in rounded fragments of an augitic glass. Orthoclase (Sanidine), abundant in worn twin crystals in the tuffs of the east of Fife. Plagioclase. Biotite. Pyrope, in the tuffs (and more rarely in the basalts) of Elie. Nigrine, common in some of the dykes, more rarely in the tuffs of Elie. Saponite, Delessite and other decomposition products. Semi-opal, one specimen found in the later (Permian?) agglomerate of Arthur's Seat. Asphalt, abundant at Kincraig, near Elie. Fragments of wood, with structure well preserved, may be included here. Dr. Heddle has described from the neck of tuff at Kinkell, near St. Andrews, large twin crystals of a glassy orthoclase, which are invariably much worn, and preserve only rudely the form of crystals. He justly remarks that they have no connection with drusy cavity, exfiltration vein, or with any other mineral, and look as if a portion of their substance has been dissolved away. Internally, however, they are quite fresh and brilliant in lustre, though sometimes much fissured.[99] [Footnote 99: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 223.]