Ixxvl FLORA ORCGADENSIS. 
largely derived from the grinding up of flagstones in 
which carbonate of lime is present in considerable 
amount, and also because they contain shells and cal- 
careous rocks that have been transported from sources 
outside the county. In this respect they resemble the 
shelly or calcareous boulder-clay of Caithness, with 
which they agree also in origin. 
ICE AGE. 
To understand the varieties of the Orkney boulder- 
clays it will be necessary to give a brief account of 
the manner in which they are formed. It has been 
clearly proved that at the climax of the Ice Age the 
north of Scotland was covered by an ice sheet several 
thousand feet thick. The North Sea also was com- 
pletely occupied by ice derived principally from the 
high land of Scandinavia. The Scotch ice was mov- 
ing slowly down the eastern slopes of the Highlands 
into the North Sea, but there it was met by the 
Scandinavian ice, which, being in far greater volume, 
overmastered it, and forced it to turn backward over 
Orkney and the north-east of Caithness to escape 
finally into the Atlantic. Orkney was accordingly 
glaciated at this period by a thick sheet of ice that 
came originally from the basin of the Cromarty Firth, 
and after passing for a distance over the floor of the 
North Sea, was impelled north-westwards across the 
islands. In its progress it brought with it fragments 
of Highland rocks, of the Secondary strata around 
Brora, in the east of Sutherland, and many varieties 
of rocks and shells from the sea bottom to the south- 
east of Orkney. These all occur in considerable 
