1883-1884.] 2^5 



remained, perhaps, for ages a soil on which a luxurious vegeta- 

 tion flourished ; or the slow accumulation of sediment in the 

 clear waters of a volcanic lake. The lignites mark even longer 

 pauses, and Dr. Jas. Geikie has shown that the important lignites 

 and true coals of the Faroes* are on one horizon, and must mark a 

 very vast interval indeed. True coals and lignites are also met 

 with in Iceland, probably on the same horizon, and Professor 

 Geikie mentions local beds of "black cherry-coal" in Mull. 

 But this is not all. A higher series of glassy trachytic lavas is 

 preserved in the northern districts of Iceland, which have no 

 representatives in Ireland, though underlying yellow tuffs with 

 plants show them to belong to the same Tertiary age. To this 

 series probably belongs the pitchstone of the Sciir of Eigg so 

 well described by Professor Geikie, t and which he considers to 

 belong to a period of volcanic activity widely separated from 

 that of the basalts. " Their successive beds, widely and deeply 

 eroded by atmospheric waste, were here hollowed into a valley 

 traversed by a river, which carried southward the drainage of 

 the wooded northern hills. Into this valley, slowly scooped out 

 of the older volcanic series, the pitchstone and porphyry coulees 

 of the Scur flowed. Vast, therefore, as the period must be 

 which is chronicled in the huge piles of volcanic beds forming 

 our basalt-plateaux, we must add to it the time needed for the 

 excavation of parts of those plateaux into river valleys, and the 

 concluding period of volcanic activity during which the rocks 

 of the Scur of Eigg were poured out." The eruptions which 

 produced a massive formation, extending, probably without a 

 break, for a thousand miles in a direct line, must thus have 

 occupied an immense time, which may not unreasonably be 

 supposed to represent an entire geological period. 



But since the close of this period of volcanic energy, — for 

 the present localized manifestations in Iceland are as utterly 

 disconnected from the basaltic outflows, as an outburst in 



* The beds of coal and accompanying shale are 6 to 7 ft. thick, and extend over 

 quite 5000 acres. Stokes, ^.J.G.S. Vol. xxxvi., p. 625. 

 t {i^uart. Journ. Geol. Soc, vol. xxvii., p. 310. 



