Vol. 52.] BASALT-PLATEAUX OF NORTH- WESTERN EUROPE. 



353 



beautifully columnar, its slender prisms curving from a central line 

 until their ends abut against the agglomerate. The truly intrusive 

 character of this basalt is 



well shown on the southern Fig. 13. — Columnar basalt invading 

 front of Coroghon M6r, and agglomerate of volcanic vent. Coro- 

 on the northern face of ghon Mdr, Isle of Canna. (Height 

 Aim an, as represented in more than 20 feet.) 

 the accompanving diagrams 

 (figs. 13 & 14). 



Although there is no 

 conclusive evidence that 

 these intrusions belong to 

 the time of the activity of 

 the vent, yet they differ 

 so much from the ordinary 

 dykes (one of which also 

 cuts the agglomerate and 

 ascends through the con- 

 glomerates and basalts 

 above), are confined so 

 markedly to the vent and 

 its immediate proximity, 

 and resemble so closely the 



basalt-injections of other vents, such as those of the Carboniferous 

 and Permian necks of Scotland, that they may with every proba- 

 bility be regarded as part of the mechanism of the Canna volcano. 



Though the form 

 and size of the vent Pig. 14. — Columnar basalt invading vol- 

 of this volcano cannot canic conglomerate. Northern side of 

 be precisely defined, Alman Islet, Canna. 

 the upper part of its v^ / ft 



agglomerate is dove- 

 tailed in the most 

 interesting way with 

 a series of coarse con- 

 glomerates, which 

 indicate strong 

 aqueous action in this 

 part of the volcanic 

 area during the time 

 of the eruption of the 

 plateau-basalts. As 

 the history of the 

 eruptions of the Canna 

 vent is so closely linked 

 with that of some 



powerful river which flowed across the lava-fields in this part of 

 Western Scotland, I reserve further account of it for the next 

 section of this paper. 



