XXIV.— Gabbro Tuff in the Central Volcanic Complex of Arran

Abstract

In the literature on the Tertiary volcanic districts of Britain there are numerous references to crushing and brecciation along tectonic planes, shattering by explosive action, and veins or dykes filled with intrusive tuffs ( cf. Bailey, et al. 1924, 345-6; Richey and Thomas, 1930, pp. 130, 136, 147, 215). A distinction between tectonic crushing and pyroclastic activity is, however, not always easily drawn. In a number of localities in Skye, Harker remarks on this difficulty. A locality to which particular reference is made is a much faulted zone following the shore to the north-east of Creag Strollamus (Harker, 1904, pp. 20, 17). Here Harker found rocks which in some places could clearly be seen to result from mechanical crushing of granite and in others were equally clearly of pyroclastic character, the fragments being of varied compositions. Yet other cases, however, did not permit of any certain conclusion. During the course of detailed mapping of the Creag Strollamus area the present writer found that undoubted pyroclastic rocks occur which are apparently unrelated to those of the main volcanic epoch in Skye, but which, in fact, post-date the gabbros and granite. The development of these late pyroclastic rocks depends primarily on the existence of a strongly faulted zone, so that all variations exist from fragmental rocks of purely tectonic origin to those formed by pyroclastic action. The recognition of the latter is aided by the observation of identical breccias or tuffs at contacts which are devoid of indications of tectonic movement (King, 1953, This 250-word extract was created in the absence of an abstract