544 Geological Society ;— 



while the heavier and more fusible basaltic materials spread between 

 the strata in vast sheets of enormous extent, which, when cooled, 

 formed dolerite and basalt. Besides the larger intrusive masses, the 

 whole district around each of the volcanic vents is traversed by a 

 wonderful plexus of dykes and veins, composed of both acid and 

 basic rocks, some of the dykes of basic composition extending to 

 extraordinary distances, as pointed out by Prof. Geikie. The great 

 igneous masses, besides disturbing the older strata through which 

 they have been forced, have effected a remarkable metamorphism in 

 them, the amount of this metamorphism and the distance to which 

 it extends being in each case proportioned to the bulk of the intru- 

 sive mass. 



From a consideration of the whole of the evidence it appears 

 highly probable that the first period of igneous activity (namely, 

 that of the eruption of felspathic lavas from the great volcanoes) 

 was contemporaneous with the Eocene sedimentary formations ; the 

 second period, that of the great and prolonged outbursts of basaltic 

 lavas from the same vents, was certainly that of the Miocene ; while 

 the third period, or that of the formation of the " puys," may, with 

 a great show of probability, be correlated with the Pliocene. 



The igneous activity during the Tertiary period in the Northern 

 Hebrides appears to have extended in all its magnitude, and to have 

 exhibited similar stages in its development, far to the southwards, 

 as is illustrated by the rocks of Arran, Antrim, and the Mourne 

 Mountains. But eveu this tract, extending 400 miles from north to 

 south, which was characterized by grand volcanic phenomena during 

 the whole of the Tertiary periods, can only be regarded as a portion 

 of the great belt of volcanoes which at that epoch extended through 

 Greenland, Iceland, the Faroe Islands, the Hebrides, Ireland, Central 

 France, the Iberian peninsula, the Azores, Madeira, Canaries, Cape- 

 Yerd Islands, Ascension, St. Helena, and Tristan d'Acunha, and 

 which constituted, as shown by the recent soundings of H.M.S. 

 ' Challenger,' a mountain-range comparable in its extent, elevation, 

 and volcanic character with the Andes of South America. The 

 admirable manner in which the relations between the Volcanic and 

 Plutonic rocks are exhibited in the old volcanoes of the Hebrides 

 renders them of special interest to the geologist ; and the further 

 illustrations of the same phenomena which are afforded to us by the 

 relics of a still older series of volcanoes, are made more clear and 

 striking by the aid of their analogies. 



The Newer Pdlceozoic Volcanoes. — In the district of Lome we find 

 a great series of old felspathic lavas which, in their lower part, 

 alternate with conglomerates and sandstones, and which, in their 

 higher portions at least, appear to be of snbaerial origin. It is 

 evident that we have here the relics of what was once a widely 

 spreading plateau made up of lava-streams, like that of Tertiary age 

 already described. These rocks were evidently formed long subse- 

 quently to the Lower Silurian strata, but before any of the Secondary 

 sediments were deposited. 

 The ccniial and eeuihern districts of Scotland exh : bit enormous 



