356 SIR A. GEIKIE ON THE TEETIAEY [May 1 896, 



It is thus clear that the loose materials of the vent were directly- 

 exposed at the surface when the conglomerate was accumulated, 

 and, indeed, that these materials served to supply some of the 

 detritus of which the conglomerate consists. The absence of any 

 trace of a cone and crater at the vent may perhaps be explicable on 

 the supposition that their incoherent material was washed down by 

 the currents that swept along and deposited the conglomerate. 



The mass of sedimentary material (b) which overlies the ag- 

 glomerate of the vent forms a conspicuous feature along the lower 

 half of the precipices at the eastern end of Canna. It rises to a 

 height of 250 to 300 feet above sea-level, and must reach a 

 maximum thickness of probably not less than 100 to 150 feet. It 

 gradually descends in a westerly direction both along the northern 

 cliffs and in the lower ground round Canna Harbour, insomuch 

 that in about a mile, owing to the gentle westerly dip of the whole 

 volcanic series, combined with the effect of a number of small 

 faults, it passes under the level of the sea. 



Great variation in the character of the detritus composing this thick 

 group of strata may be observed as it is followed westward. On the 

 cliffs below Compass Hill, as represented in fig. 15, p. 355, a coarse 

 conglomerate with waterworn stones, hardly to be distinguished 

 from the volcanic agglomerate of the vent, shows more or less 

 distinct bedding, or at least a succession of coarser and finer bands. 

 Towards its base it encloses numerous pieces of Torridon Sandstone, 

 sometimes subangular, but often so well and smoothly rounded as 

 to show that they must have been long subjected to the action of 

 moving water. It is further observable that, while in the agglo- 

 merate the volcanic stones have rough surfaces, those in the 

 conglomerate begin to show increasing evidence of attrition, until, 

 as the deposit is traced upward, they become almost as well rounded 

 and waterworn as the non-volcanic stones which have come from 

 another district. 



Yet amidst and overlying these proofs of transport from some 

 little distance lie abundant huge slags and blocks of amygdaloidal 

 lava, sometimes closely aggregated, sometimes scattered through 

 a volcanic tuff or ashy sandstone. The composition and structure 

 of these stones, and the manner of their dispersion through the 

 deposit, leave little doubt that they were ejected from the vent. 

 We are thus confronted with the interesting fact that, while the 

 materials of the volcanic cone were being washed down by running 

 water, eruptions were still taking place. But by degrees these indi- 

 cations of contemporaneous volcanic activity disappear. The detrital 

 materials become coarser and more distinctly water-rolled, until they 

 pass into greenish sandstones and fine conglomerates. Yet the 

 matrix even of these higher sediments is largely composed of fine 

 volcanic detritus, and probably points to occasional discharges of 

 dust and ashes. 



Yarious sills or intrusive sheets have been injected into this sedi- 

 mentary group along the precipices at the eastern end of Canna, and 

 form there lenticular bands. One of these (c) is si own in fig. 15. 



