Vol. 52.] BASALT-PLATEAUX OF NORTH- WESTERN EUROPE. 303 



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 washing of 

 vegetable matter into the pool, which was finally buried under 

 another outflow of basalt. 



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 south of the headland of Ceann an Eilein. It doubt- 

 less 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 as we proceed 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 Dim M6r and 

 Dun 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 Dun Mor, 

 which is represented in PI. XVII. The lower band, 4 or 5 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 seaward 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 some- 

 what 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 that they both rapidly change their charac- 

 ters and disappear. The lower, though formed of coarse detritus 

 under the Dun Mor, passes on the opposite cliff, in a space of not more 

 than 60 yards, into fine tuff and shale, about 6 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 Dun Beag, 

 and again show the usual coaly layers at the top. On the eastern side 

 of the gully in the coast, about 160 yards north-east of Dun Mor, 

 the same band is reduced to not more than 3 feet in thickness, 

 consisting chiefly of fine conglomerate, wherein well waterworn 

 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 eastward and westward, 

 in the cliff opposite the Dun Mor, dwindling down at last to 



