226 
THE TERTIARY VOLCANOES 
BOOK VIII 
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 ol 
basalt. 
It is on the southern coast of the isle of Sanday that the higher inter- 
calations of sedimentary material among the basalts are most instructively 
Fig. 271. — Dun Mur, Sanday. (From a photograph by Miss Thom.) 
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 Dun Mbr and Dun Beag (Big and Little Gull 
Bocks), which here rise from the foreshore, two distinct platforms of detrital 
