Fig. 270.—Section of shales and tuffs, with a coniferous stump lying between two basalt-sheets, Cùl nam Marbh, Canna.
Rather less than half a mile towards the south, on the roadside at the gully of Cùl nam Marbh, the basalts enclose a sedimentary interstratification which not improbably lies on the same horizon as those just described along the northern shore. The relations of the rocks at this locality are shown in [Fig. 270]. A remarkable slaggy basalt (a) rises into a hummock, against which have been deposited some fine granular tuffs (b) whereof only a few inches are visible, that pass up into a thin band of dark shale (c), including a layer of pebbly ferruginous tuff, with small rounded pea-like pieces of basalt, basic pumice, bole, limonite, etc. At the top of this shale an irregular parting of coaly material (d) lies immediately under the slaggy base of the succeeding basalt (e). It will be observed that this upper lava cuts out the shale and thus comes to rest directly upon the lower sheet. At the point where it begins to descend it has caught up and enclosed a small tree-stump (d′) which stands upright on the coaly parting and shale. This stump, at the time of my visit, measured five inches in height by three inches in breadth; it had been thoroughly charred and was crumbling away on exposure, but among the pieces which I took from it sufficient trace of structure can be detected with the microscope to show the tree to have been a conifer.
We have here another instance of the deposition of volcanic dust and fine mud in a pool that filled a hollow in the lava-field. Again we see that the closing act of sedimentation was the subsidence of vegetable matter in the pool, which was finally buried under another outflow of basalt.
Fig. 271.—Dùn Mòr, Sanday. (From a photograph by Miss Thom.)
It is on the southern coast of the isle of Sanday that the higher intercalations of sedimentary material among the basalts are most instructively displayed. At the eastern end of this island, as already stated, the lowest and coarsest conglomerate is visible on a skerry immediately to the south of the headland Ceann an Eilein. It doubtless underlies the Sanday cliffs, but is not there visible, for the basalts descend below sea-level. These volcanic sheets have a slight inclination westward; hence in that direction we gradually pass into higher parts of the series. In the Creag nam Faoileann (Seamews' Crag) and the gully that cuts its eastern end, likewise in the two singularly picturesque stacks of Dùn Mòr and Dùn Beag (Big and Little Gull Rocks), which here rise from the foreshore, two distinct platforms of detrital material may be noticed among the basalts. Both of these can be well seen on Dùn Mòr, about 100 feet high, which is represented in [Fig. 271]. The lower band, four or live feet thick, is here a rather coarse conglomerate which lies upon a sheet of scoriaceous basalt that extends up to the base of the Creag nam Faoileann. It is directly overlain by another basalt, about 30 feet thick, which dips seawards and forms a broad shelving platform, whereon the tides rise and fall. On this stack a second coarse conglomerate, about 10 feet thick, forms a conspicuous band about a third of the height from the bottom; it is composed mainly of well-rounded blocks of various lavas up to 18 inches or more in diameter, but it contains also pieces of Torridon Sandstone. It is covered by about 60 feet of basalt, which towards the base is somewhat regularly columnar, but passes upward into the wavy, starch-like, prismatic structure.
If now we trace these two intercalated zones of conglomerate along the shore, we find them both rapidly to change their characters and to disappear. The lower, though formed of coarse detritus under the Dùn Mòr, passes on the opposite cliff in a space of not more than 60 yards, into fine tuff and shale, about six feet thick, which become carbonaceous at the top where they are overlain by the next basalt. A hundred yards to the east, the band likewise consists of tuffs and ashy shales, which underlie the basalts on the Dùn Beag, and again show the usual coaly layers at the top. On the east side of the gully in the coast, about 160 yards to the north-east of Dùn Mòr, the same band is reduced to not more than three feet in thickness, consisting chiefly of fine conglomerate, wherein well water-worn pebbles of Torridon Sandstone and epidotic grit appear among the predominant volcanic detritus. This conglomerate is surmounted by a few inches of dark carbonaceous mudstone or shale. Rough slaggy basalts lie above and below the band.
The upper conglomerate dies out, both towards the east and the west, in the cliff opposite to Dùn Mòr, dwindling down at last to merely a few pebbles between the basalts. It lies in a kind of channel or hollow among these lavas. This depression, in an east and west direction, cannot be more than about 65 yards broad.
Probably still higher in the series of basalts is another intercalation of sedimentary layers which may be seen in the little bay to the east of Tallabric, rather more than a mile to the west of the Creag nam Faoileann. It rests upon a coarsely slaggy amygdaloid, and is from six to ten feet in thickness. The lower and larger part of the deposit consists of greenish pebbly sandstone and fine conglomerate, largely composed of basaltic detritus, but including abundant well-smoothed and polished pebbles of Torridon Sandstone, green grit, quartzite, etc. The stones vary from mere pea-like pebbles up to pieces two or three inches long, the largest being generally fragments of slag and amygdaloid which are less water-worn than the sandstones and other foreign ingredients. The uppermost two or three feet of the intercalation consist of dark carbonaceous mudstone or shale, made up in large measure of volcanic detritus, which may have been derived partly from eruptions of fine dust, partly from subærial disintegration of the basalt-sheets. Some layers of these finer strata are full of remains of much macerated plants.
Other thin coaly intercalations have been observed among the basalts of Canna, some of which may possibly mark still higher horizons than those now described. But, confining our attention to the regular sequence of intercalations exposed along the Sanday coast, we find at least four distinct platforms of interstratified sediment among the plateau-basalts of this district. Each of these marks a longer or shorter interval in the outflow of lava, and points to the action of moving-water over the surface of the lava-fields.