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FENNELL : SOME PHYSICAL FEATURES IN ICELAND. 
I have only seen one explanation of this peculiarity, and that 
is given by the Rev. H. J. Johnson-Lavis, M.D., and he gives 
the following reasons : — 
(1) Irregular lichen covering. 
(2) Grasses in separate patches. 
(3) Prominent tufts entrap more dust. 
(4) If soil is boggy, grass grows best on top of humps. 
(5) Swarms of birds fertilise, by their droppings on the 
top of the hump. 
The first two reasons, I think, deal with effect rather than 
cause, and the last three no doubt each give their small quota 
to the result, some more some less, but I cannot help thinking 
that the real and primary cause of the formation of the tussocks 
is unmentioned. 
I have drawn a small diagram (Fig. 1) which explains my 
view, that the tussocks or humps are formed much more by the 
alternation of temperature than by anything else. 
Fig. 1. 
Supposing we imagine the ground in its original level condi- 
tion undrained and full of moisture. In winter the severe frost 
comes, freezing the ground hard to a depth of 4 or 5 feet, in 
summer a certain proportion of this frozen ground is thawed, 
with a band of frozen ground left between the thawed surface 
and the unfrozen soil below, which is probably well charged with 
moisture. At certain points it is highly probable that the frozen 
band will be weakened and cracked, and through such holes or 
fissures the softer earth will be forced up, probably by the 
