lviii FLORA ORCADENSIS. 
Nowhere have I seen plants stunted as in North 
Ronaldshay. On the tops of the higher hills it is 
also well defined. Geum rivale is a tall plant of 
14 or 2 feet. One had somehow strayed half-way 
up the Ward Hill, Hoy, and was dwarfed and 
creeping. St. John’s wort, Lotus corniculatus, and 
others are met with on exposed surfaces, and look 
as if they were creeping plants. 
We need say little about rainfall, as it is seldom 
plants suffer from want of moisture. The annual 
rainfall is not excessive—37 inches—and is low 
compared with the west of Scotland and some of 
its higher regions. The supply of moisture, especi- 
ally in summer, depends more on the manner of 
precipitation than the quantity. Drizzly rains, as 
we frequently have, and fogs, keep vegetation well 
supplied with this sine qua non of plant life. 
PEAT BOGS. 
Peat is a heterogeneous mass of dead _ plants. 
The layers are composed of the remains of different 
plants which for the time being were dominant. 
They accumulated under varying conditions which 
prevailed over long periods of time back to the 
Great Ice Age. The deeper peat bogs, found gener- 
ally in valleys, were formed where marsh conditions 
prevailed. The plants which constituted the for- 
mation were chiefly monocotyledons—aquatic plants, 
sedges and grasses. A little mud accumulated with 
these, and on the soil so formed plants characteristic 
of the bog grew—e.g., cotton grass, sphagnum, carices, 
rushes, &c. So the process of accumulation goes 
