16 ON FARLETON FELL 
from which plants appeared to be absent. This ques- 
tion of deserts—that is, of areas of the Earth’s surface 
where the prevailing mantle of vegetation is wanting 
—is an interesting one, and may fittingly detain us 
for a few minutes. Deserts are produced by the 
failure of one or more of the conditions which are 
necessary for plant life. The factors in question may 
_ be briefly defined as temperature, light, water, atmo- 
sphere, and mineral salts. The majority of the higher 
plants have developed a complicated root-system 
for the purpose of collecting water (containing 
salts) from the soil, and of anchoring the organism 
firmly in its chosen abode, so a soil is also usually 
essential. Here on Farleton Fell soil is missing over 
much of the surface, which is occupied by naked 
limestone rock. The absence of soil is due to the fact 
that the material—carbonate of lime—of which the 
rock is composed is soluble in water, unlike, for in- 
stance, the materials of which slate or sandstone rocks 
are composed; the rains slowly dissolve it, and it 
passes in solution down through crevices in the strata, 
leaving behind only a small insoluble residue. This 
residue, where not also washed away, collects in every 
little hollow, and lowly plants such as Algze and 
Mosses soon discover it and colonize it. Their de- 
cayed remains add nutritive material to the little 
pocket, and help to retain water, and thus prepare 
the way by degrees for higher forms of life; till at 
length the crevices become filled with a luxuriant 
vegetation which, as we shall see later, is of a rather 
peculiar type. It should be noted that even the bare 
rock is not so inhospitable as completely to exclude 
plant life. If we examine it with a lens we shall see 
