THE GEOLOGY OF ORKNEY. XCV 
and glaciers filled many of the larger valleys. The 
Lower Turbarian, or period of the hundred-foot beach, 
was on the whole colder than the Upper Turbarian 
period. 
All these changes must have had a very marked 
influence on the distribution of the plants in Orkney 
and the north of Scotland generally, and a great part 
of the history of the flora of the county lies stored up 
in the peat beds and Forest Beds, and will some day 
be more completely known than at present. The 
study of the peat-mosses, in fact, should show us the 
stages by which our flora has reached its present 
condition, and enable us to decide which plants are 
natives and which have been introduced by commerce 
or otherwise in recent times. It may also be possible 
to establish that many southern plants once common 
in Orkney are now absent, or can maintain their hold 
only where specially protected and encouraged, while 
on our highest hill-tops still persist the remnants of 
a flora that once covered a great part of the low 
plains, where now a different type of vegetation 
prevails. 
