70 Notices of Memoirs — 



have rarely flowed to a distance of more tban ten miles from the 

 volcanic vent, those of basaltic character often extended to distances 

 of fifty or sixty miles, or even more. The same difference of 

 behaviour of the two classes of lava has often been remarked in the 

 case of existing volcanos. 



Besides the volcano of Mull, we have evidence of the existence of 

 four other great vents in the northern part of the Hebrides, namely, 

 in the peninsula of Ardnamurchan, and in the islands of Eum, 

 Skye, and St. Kilda respectively. In each of these a period charac- 

 terized by the eruption of felspathic lavas was followed, after a con- 

 siderable interval, by one during which nearly all the materials 

 thrown out were of basaltic composition. The volcano of Mull is in 

 a far more perfect state of preservation than the others, owing to 

 the great amount of central subsidence which has taken place in its 

 mass. This central subsidence appears to be strictly analogous to 

 that which has been shown by Mr. Darwin, Mr. Heaphy, and Krug 

 von Nidda, to have occurred in the case of recent volcanos in the 

 Cape de Verde Islands, New Zealand, and Iceland. From an ex- 

 amination of the areas covered by the great Tertiary volcanos of the 

 Hebrides, and of the interesting data afforded by the present posi- 

 tions of their lava-streams, etc., we are able to estimate that v/hile 

 the volcanos of Mull and Syke were certainly of far greater bulk 

 than Etna, those of Ardnamurchan, Eum, and St. Kilda could have 

 been of scarcely inferior dimensions. 



There is proof that after the extinction of the five great volcanos 

 of the Northern Hebrides and the very extensive denudation of the 

 great plateaux composed of their lavas, there burst out a number of 

 sporadic eruptions which resulted in the formation of comparatively 

 small volcanic cones, analogous to the " puys " of Central France, 

 which have been so admirably described by Mr. Poulett-Scrope. 

 These " puys " of the Hebrides are very numerous, and are exhibited 

 to us in various stages of preservation. 



The formation of the various volcanic piles of the Western Isles 

 was accompanied by the intrusion of innumerable igneous masses of 

 all sizes among the surrounding older strata. The liquefied rocks 

 of acid composition accumulated in great lenticular masses in the 

 midst of the stratified rocks, consolidating into felsite and granite ; 

 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 ease proportioned to the bulk of the intru- 

 sive mass. 



From a consideration of the whole of the evidence it appears 



