THE GEOLOGY OF ORKNEY. lxxix 
the data afforded by the district itself, and what is 
known of the history of the adjacent parts of Scot- 
land ; the lapse of time since the deposition of the Old 
Red Sandstone is so enormous, and so little evidence 
is available regarding the Secondary and Tertiary 
rocks in north-eastern Scotland, that we are very 
much in the dark regarding many chapters in the 
history of the islands. We know, for example, that 
an extensive land surface existed in the region 
between the period at which the flagstones were laid 
down and that to which the Upper Old Red Sand- 
stune of Hoy belongs. Even before this there was a 
land surface of which the only trace now left in our 
area is the unconformability exhibited at the base of 
the Stromness conglomerates. But these physical 
features are long anterior to the development of the 
existing fauna of the archipelago, and do not remotely 
concern us in this connexion. 
The most obvious feature of Orcadian topography 
is the existence of groups of islands with intervening 
sounds and straits, and the highly irregular con- 
figuration of the coasts. The beds of sandstone and 
flagstone in the different islands must at one time 
have been continuous, and their separation has 
accordingly to be attributed to some agent of erosion, 
which, by removing great quantities of rock, has 
excavated the valleys, sounds, and bays. It is fairly 
obvious that this cannot have been done by the sea, 
as marine erosion is practically absent in the sheltered 
bays; and, in fact, deposition is going on there fairly 
rapidly in some places, as is known, for example, by 
the necessity of dredging scme of the harbours in 
