470 Geikie — Volcanic Rocks of Great Britain. 



their base a thick group of dark reddish-brown amygdaloidal por- 

 phyrites and tuffs. Connected with these rocks are numerous bosses 

 of a coarse volcanic agglomerate, which descend vertically through 

 the coal-measures altering the coal. They are the "necks," or 

 orifices, from which was ejected the volcanic material which now 

 forms a conspicuous range of rising grounds overlying the heart of 

 the coal-basin of Ayrshire.^ 



New Bed Sandstone. — The New Eed Sandstone series of Devon- 

 shire, in the neighbourhood of Exeter, furnishes clear proofs of 

 volcanic activity. Sheets of a dark reddish-brown feldspathic rock, 

 sometimes compact or porphyritic, but usually of scoriaceous 

 charactei', are intercalated among the lower parts of the Eed Sand- 

 stone series of that neighbourhood. That these are not intrusive 

 masses, but belong to the same geological period as the Eed Sand- 

 stones themselves, is shown by the occurrence of fragments from 

 them in the overlying conglomerates. Sir Henry De la Beche, who 

 described these igneous rocks many years ago, noticed that the more 

 compact portions, instead of extending horizontally as beds among 

 the sedimentary strata, descend vertically through them, as if these 

 detached parts marked the site of some of the orifices whence the 

 melted lava was erupted.'^ 



The series of successive volcanic phenomena, which may thus be 

 traced through the Palaeozoic rocks of the British Islands up to the 

 New Eed Sandstone, is now abruptly broken. I am not aware of 

 any satisfactory proofs of contemporaneous volcanic rocks among 

 the Secondary rocks of Britain, save in the Eed Sandstone of 

 Devonshire just referred to. Following a suggestion of Professor 

 Edward Forbes, I formerly regarded the great trappean masses of 

 Skye, and the other Western Islands, as probably of Oolitic age. 

 But more recent investigations in Antrim, Mull, and Eigg, have 

 convinced me that in these districts, and probably also in Skye, the 

 great basaltic plateaux, which form so conspicuous a feature in the 

 scenery of our north-western sea-board, date from Tertiary times.'^ 

 As the importance of these later volcanic phenomena in the general 

 geology of the country is not, perhaps, adequately understood, I 

 may be permitted to refer to this part of the subject at somewhat 

 greater length. 



Tertiary. — From Antrim northwards through the inner Hebrides 

 and the Faroe Islands to Iceland there is a broken chain of volcanic 

 masses, part, and not improbably the whole, of which date from the 

 Miocene period. In Ireland sheets of dolerite and basalt, in all 500 

 or 600 feet thick, and some 1200 square miles in extent, repose 

 directly upon an eroded surface of Chalk. In Mull similar plateaux, 

 overlaid with masses of porphyrite and trachyte-like rocks, attain a 

 united thickness of more than 3000 feet, yet at their base they con- 



^ See Geikie, Geol. Mag., Vol. I. for June, 1864, p. 22. 



* Sir H. De la Beche, Devon and Cornwall, p. 199. See, also, Conybeare and 

 Phillips, Geol. England and Wales, p. 294. 



3 See E. Forbes, Quart. Journ. Geol. Soc, vol. viii. p. 108; Geikie, Trans. Roy. 

 Soc, Edin., vol. xxii. p. 649 ; and Proc. Roy. Soc, Edin., 1866-67. 



