Note 395 — Chapter XLVI · TYPES OF STRUCTURE IN THE ACID ROCKS—BOSSES

Many years ago I was much struck with the evidence of alteration in the igneous rocks of Mull, and referred to it in several papers, Proc. Roy. Soc. Edin. (1866-67) vol. vi. p. 73; Quart. Journ. Geol. Soc. xxvii. (1871) p. 282, note. The subject was more fully discussed in my memoir in the Trans. Roy. Soc. Edin. vol. xxxv. (1888) p. 167, from which the account in the text is taken. Prof. Judd has more recently referred the alteration to solfataric action (Quart. Journ. Geol. Soc. xlvi. 1890, p. 341). As already mentioned, I have been unable to detect evidence of such action. The alteration is always intimately connected with the presence of intrusive masses, and it affects indifferently any part of the basalt-plateaux which may chance to lie next to these masses. The bedded lavas can be traced step by step from their usual unaltered condition in the plateaux to their metamorphosed state next to the eruptive rocks. The nature or degree of the metamorphism has doubtless somewhat varied with the composition and structure of the rocks affected, and with the character and mass of the eruptive material; but it is certainly not confined to the older parts of the plateaux, nor to any supposed pre-basaltic group of andesites. I have found no evidence that such a group anywhere preceded the plateau-basalts. The andesites, so far at least as my observations go, were erupted at intervals during the plateau period, and alternate with the true basalts. The greatest accumulation of them lies not below but above the general body of the basalts, in the "pale group" of Mull. Nor even if the term "propylite" be adopted for these altered rocks, can it be applied to any special horizon in the volcanic series. The alteration of the basic rocks by the granophyre of St. Kilda will be described in the account of that island in Chapter xlvii.