lxxx FLORA ORCADENSIS. 
order to prevent the accumulation of silt. There can 
be no doubt that in places the sea is actually attack- 
ing the land, as, for example, on the exposed west 
coast, especially in the Mainland and in Hoy, but this 
action produces a simple type of coast line with few 
indentations, singularly unlike the complex and 
sinuous coast that prevails in the islands as a whole. 
We must also allow that in sounds where there are 
rapid tides a considerable amount of scour may be 
going on which not only prevents the accumulation of 
sediment, but may be deepening the channels even 
where these consist of solid rock. This tidal erosion, 
however, is confined to a very few localities, and has 
taken only a small part in the development of the 
present configuration of the islands. 
If we compare Orkney with Caithness we can see 
that all the main features of the two counties are 
strikingly alike. The low, gently swelling hills, with 
broad, open valleys between, and great flat plains at 
their feet, that characterise the north-east of Caith- 
ness, have their exact counterparts in Orkney. In 
Caithness we have a great plain which has been 
carved out of the soft flagstone almost entirely by 
the action of the atmosphere, weathering, rain and 
streams, and there is no reason to believe that the sea 
has taken an important part in its formation. 
Similarly in Orkney the scenery is essentially that of 
a great plain, but all its lower parts have been flooded 
by the sea. The bays and sounds are drowned 
valleys, and in the gentle submarine slopes and the 
long stretches of shallow water that girdle the islands, 
we have the low, flat plain which in Caithness lies at 
