lxxxiv FLORA ORCADENSIS. 
beach, and that upheaval: has taken place in succes- 
sive stages. The absence of these beaches in Orkney 
goes to prove that the islands have at no time since 
the Ice Age been more deeply submerged than at 
present, and that the extent of dry land now is as 
small as it has ever been since the ice sheets melted 
away. It is quite easy for any one familiar with the 
islands to see that this must be the case. In many 
places, as, for example, between Kirkwall and Scapa. 
there are necks of low land that, if once covered by 
the sea, would have been converted into sounds 
through which powerful tidal currents would course. 
These currents would have scoured out all sand, clay, 
and mud, and left a bare floor strewn with large 
rocks, and fringed with gravel beaches. But no such 
valleys can be found in Orkney at present, and we 
may be quite sure that the sea has never invaded 
these grounds. There is indeed some evidence to 
show that subsidence is still going on, but this is not 
satisfactory ; and if it is in progress it is certainly 
very slow. 
We cannot assume, however, that Orkney has 
never been more extensive since the melting of. the 
ice sheets than it is now. As a matter of fact, there 
are very strong reasons which would lead to the 
opposite conclusion. The raised beaches merely prove 
that the land was at one time submerged and has 
risen in stages. At each halt in the process of 
elevation a beach platform was formed. It is quite 
conceivable, however, that the movement was oscil- 
latory, and that, for example, after the hundred-foot 
submergence the land rose considerably, and then 
