lxxiv FLORA ORCADENSIS. 
stone soils are often modified by the presence of peat. 
The flagstone soils, however, are always thin, and the 
rocky subsoil is generally only a few inches below the 
surface. This ensures good drainage, but is in other 
respects a drawback, and neither for a cultivated nor 
for a natural flora do these soils prove very productive. 
The next in importance to flagstone among the 
Orkney rocks is sandstone, which occurs mainly in 
Eday, Burray, South Ronaldshay, and Hoy. The 
John O’Groat’s sandstones are always calcareous, and 
they are sometimes stained dark red with iron oxide. 
The soils derived from the weathering of these sand- 
stones are naturally more porous than those formed 
on the flagstones, and they seem to be of inferior 
fertility, though they cover no great area except in 
Burray and Eday, where a large part of the sandstone 
districts is left uncultivated, and gives rise only to a 
rank growth of heather. 
In Hoy also the Upper Old Red Sandstone seems 
to produce a barren soil, for over its whole extent 
there is very little cultivation. This may, however, 
be partly due to the elevation of the ground, as the 
Upper Old Red there forms a range of flat-backed 
hills, most of which is above the altitude to which 
cultivation is carried in Orkney. The same character- 
istics mark the area of these rocks that occurs at 
Dunnet Head, in Caithness. 
The granite and gneiss of Stromness, and the 
voleanic rocks of Deerness and the North Isles, 
occupy so small a part of the surface of the county 
that it is hardly necessary to consider the peculiar- 
ities of the soils that arise from them. 
