116 



DR OEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Islands have been repeatedly copied, and have served as typical figures of intrusive 

 igneous rocks. 



Though none of the examples in the Inner Hebrides attain the dimensions of the Fair 

 Head sheet, still they present a much greater variety of rock and of geological structure 

 than is to be found in Antrim. I have already referred to the base of the thick, overlying, 

 basalt-plateaux in Scotland, as a horizon along which a prodigious quantity of eruptive 

 material has subsequently been injected. Part of this material consists of basic rocks 

 in the form of dykes, veins, or sills ; part of it is included in the acid group, and 

 comprises veins, sheets, and bosses of granitoid, felsitic, rhyolitic, trachytic, and pitch- 

 stone rocks. With regard to the basic sheets (dolerites, basalts, &c.) which occur on this 

 horizon, I would remark that while in western Scotland the Antrim type is also found, 

 the vast majority of the sheets belongs to a quite distinct type. For the sake of 

 continuity, I may first describe some examples of the occurrence of thick, coarsely 

 crystalline, rapidly diminishing sills like those of the north coast of Antrim. 



Fig. 34. — Section at Farragandoo Cliff, west end of Fair Head, showing the rapid splitting up and dying out of an Intrusive 

 Sheet, a, Carboniferous sandstone ; b, Carboniferous shale ; c, intrusive sheet. 



On the coast of Skye, between Lochs Slapin and Eishort, the prominent headland of 

 Suisnish has long been known to geologists from the section of it given by Macculloch 

 as an instance of the connection between overlying rocks and dykes. I have already 

 alluded to it in that relation, and refer to it again as an example of one of the thicker 

 intrusive sheets of the Inner Hebrides. Denudation has proceeded so far in that district 

 of Skye that the whole of the volcanic plateau has been stripped off, and we have only 

 some of the underlying sills left, with the platform of older rocks between which and the 

 vanished basalts they were injected. Most of these sills consist of granophyres belonging 

 to the acid group of rocks to be afterwards described. But among them there occur true 

 dolerite sheets not infrequently interposed between the granophyres and the subjacent 

 Lias, and sometimes even intercalated in the former rock. Though at first sight it might 

 be thought that these sills had insinuated themselves after the eruption of the granophyre, 



