Mr. J. W. Judd on the Secondary Rocks of Scotland. 543 



These were accompanied by the intrusion among the surrounding 

 strata, which they greatly metamorphosed, of great masses of fluid 

 rock of acid composition, and the extrusion among the other igneous 

 products of similar liquefied materials, which consolidated into 

 felsite and granite. The great volcanic mountains thus formed 

 appear to have remained in a state of comparative quiescence for a 

 vast period, during which they were subjected to great denudation, 

 and then through their midst were forced great masses of fluid 

 basaltic rocks, which continued to flow at intervals during enormous 

 periods, and gave rise to streams of basalt which accumulated to the 

 thickness of many hundreds and even thousands of feet. The great 

 intrusive bodies of this same rock consolidated into mountain masses 

 of gabbro and dolerite. "While the earlier felspathic lavas appear to 

 have rarely flowed to a distance of more than ten miles from the vol- 

 canic 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 volcanoes. 



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 Rum, 

 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 volcanoes in the 

 Cape-Yerd Islands, New Zealand, and Iceland. From an exa- 

 mination of the areas covered by the great Tertiary volcanoes of the 

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

 tions of their lava-streams &c, we are able to estimate that while 

 the volcanoes of Mull and Skye were certainly of far greater bulk 

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

 been of scarcely inferior dimensions. 



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

 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 Prance, 

 which have been so admirably described by Mr. 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 : 



