358 SIE A. GEIKIE ON THE TEETIART [May 1 896,. 



it westward, we find that it forms the depression in which the 

 sheltered inlet of Canna Harbour lies. It is exposed along the 

 shores and also in the islands enclosed within the same bay. But 

 it is not traceable farther west, possibly because it seems to sink 

 beneath the level of the sea. To the south-east, though it is there 

 likewise for the greater part concealed under the waves, it rises above 

 them in one or two parts of the coast-line of Sanday, particularly 

 at the Uamh Kuadh or Red Cave, and likewise on a surf-beaten 

 skerry off Ceann an Eilein, the highest part of the Sanday cliffs — a 

 distance of about 1| mile from Compass Hill. Throughout this 

 space it retains its remarkably coarse character, and is mainly made 

 up of volcanic material. 



The numerous sections exposed in Canna Harbour enable us to 

 study the composition and local variations of this curious deposit. 

 On the northern side of the basin, while the lower part of the 

 sedimentary series continues to be an exceedingly coarse volcanic 

 conglomerate, it passes upward into finer conglomerates, tuffs, and 

 shales. In front of Canna House the imbedded blocks are of large 

 size, occasionally as much as 3 or 4 feet in diameter. They are 

 still more gigantic on Eilean a' Bhaird, where I found one to 

 contain 150 cubic feet in the exposed part, the rest being still 

 imbedded in the matrix. As they are generally somewhat rounded, 

 here and there markedly so, most of these stones have probably 

 undergone a certain amount of attrition in water. The great 

 majority of them, and certainly all those of larger size, are pieces 

 of basalt, dolerite, andesite, red bole, etc. Among them huge 

 blocks of amygdaloid and coarsely vesicular lava are specially 

 abundant. Some of these look like pieces of slag torn from the 

 upper surface of lava-streams ; others, displaying a highly vesicular 

 centre and a close-grained outer crust, are suggestive of bombs. 

 It is interesting to note here again that the amygdaloidal blocks 

 present their zeolitic infiltrations so precisely like those of the 

 amygdaloids of the plateaux, that it seems reasonable to suppose 

 the carbonate of lime, zeolites, etc. to have been introduced before 

 the blocks were imbedded in the conglomerate. 



The whole aspect of this deposit is eminently volcanic. It looks 

 like a vast sheet of lava-fragments swept away from one or more 

 cones of slags and cinders, or from the rugged surface of a lava- 

 stream. Where the vesicles were still empty, the large boulders 

 could be more easily swept along by moving water. But a powerful 

 current must have been needed to transport and wear down into 

 more or less rounded forms blocks of basic lava, many of which 

 must weigh several tons. The large block on Eilean a' Bhaird, 

 for instance, probably exceeds 12 tons in weight. 



Besides the obviously volcanic contents of the conglomerate there 

 occur here also, as in the Compass Hill cliffs, abundant pieces of 

 Torridon Sandstone. These stones are notably smaller in size and 

 more perfectly waterworn and even polished than the blocks of 

 lava. Obviously they have travelled farther and have undergone 

 more prolonged attrition. 



The matrix of the rock consists essentially of the fine detritus of 



