:352 SIE A. GEIKIE ON THE TERTIARY [May 1 896, 



plateaux that, as already remarked, we must believe them to have 

 been already filled by infiltration before the disruption of the rocks 

 by volcanic explosions, Other blocks are true bombs, with a fine- 

 grained crust outside and a more cellular texture inside, the vesicles 

 of the outer crust being sometimes dragged round the surface 

 of the stone. The variety of materials included among the ejected 

 blocks and the abundance of pieces of the red bole which so 

 generally separates the plateau-basalts indicate that a considerable 

 thickness of bedded lavas has probably been broken through by the 

 vent. Besides the volcanic materials, occasional angular pieces of 

 red (Torridon) sandstone may be observed in the agglomerate. 

 The paste is a comminuted mass of the same material as the 

 blocks, tolerably compact, and entirely without any trace of strati- 

 fication. 



The actual margin of this vent has nowhere been detected by me. 

 We never reach here the base of the volcanic series, for it is sunk 

 under the sea-level. On the other hand, the upper limits of the 

 agglomerate have been partially effaced or obscured by the thick 

 conglomerates which overlie it. There can be no doubt, from the 

 breadth of ground across which the agglomerate can be followed 

 along the shore, that the vent must have been one of somewhat 

 exceptional size, perhaps not less than -J mile in diameter, unless, 

 indeed, there were more than one in close proximity. That 

 it continued in vigorous eruption may be judged from the amount 

 of material ejected from it, the large size of its blocks, and the 

 distance to which they were sometimes thrown. 



The pieces of Torridon Sandstone were no doubt derived from the 

 extension of that formation underneath Canna. On the opposite island 

 of Rum these pre-Cambrian red sandstones are copiously developed. 

 They form there a platform through which the Tertiary volcanic 

 series has been erupted. Several remaining outliers of the bedded 

 basalts on thewestern side of that island show that the basalt-plateau 

 of Small Isles once covered that area, and that it rested imme- 

 diately on the inclined edges of the Torridon Sandstone. Probably 

 the same structure stretches westward under Canna and Sand ay. 

 No traces of any Jurassic strata have been detected beneath the 

 volcanic rocks of Rum, though they are so well developed a few 

 miles to the east in the island of Eigg. Either they were not 

 deposited over the pre-Cambrian rocks of Eum, or they had been 

 removed from that ancient ridge before the beginning of the 

 Tertiary volcanic period. Certainly I have not detected a single 

 recognizable fragment of any Jurassic sedimentary rock in the 

 agglomerate of Canna. 



This Canna vent exhibits, better than is usually shown, the 

 occurrence of dykes and irregular injections of lava through the 

 agglomerate. A large mass of a finely columnar basalt ascends 

 from the beach at Garbh Asgarnish. A similar rock forms several 

 detached crags a little farther south, particularly in the headland 

 of Coroghon Mor and the island of Alman. Here the basalt is 



