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TERRESTRIAL AND FRESHWATER ANIMALS. 179 
these run well up into the chalky upper portion, and the bed is 
clayey throughout, it is necessary that the material dug out should 
be washed in a mill to separate out the phosphate, which is required 
for commercial purposes. This mill consists of a circular pit into 
which the rough material is thrown with a constant supply of water. 
An instrument like a harrow is drawn round and round by a horse 
working at the end of a beam, as we commonly see in farmyards 
and brickyards. The muddy water is let off from time to time along 
a wooden cutter generally supported on upright poles, and is conveyed 
to a convenient place where it is received into large tanks ; the water 
is allowed to evaporate, and the fine mud, which is locally known as 
“slurry,” is covered with mould and the land restored to cultivation. 
The sandy portion generally settles along the trough, which has to 
be regularly cleared out to get rid of it. The result is, that what 
settles in the tanks 1s the finest calcareous mud, most insinuating | 
when thin, most tenacious when thick, as the inexperienced who 
have got in on foot or on horseback, when the mould has been 
newly spread over an unconsolidated bed of it, know to their cost; 
as, also, do many other land-animals which drop from the trees or 
unwittingly hop on to it; as do also even the water-beetles which 
_plep down on to it, taking it for a pond; as do the earth-worms 
which try to get through it, or the various tribes of pond-life which 
find the water disappearing and no outlet for them. 
As the thicker central portion sinks most, and the margins dry 
first, we can see the kind of track made by each creature when he is 
in the softest slime, and trace it on till he gets to the dried portion 
where he leaves little or no mark; and we find that many very 
different tracks must be referred to the same animal. 
Other phenomena producing peculiar marks or modifying the 
tracks can be well studied on the “slurries,” where accidental circum- 
stances, depending upon the season, temperature, and so on, can be 
watched, and many of these offer suggestions for further experiment. 
In the first place we must notice that although the mud poured 
into the tanks is exceedingly fine, there are degrees of fineness, and 
the less fine settles first, so that the deposit, when seen in section, is 
often finely laminated, the thickness of the lamine depending upon 
the proportion of the area of the tank to the body of water let off 
each time from the washing-mill. By splitting the clay along 
these laminze we can see successive series of tracks belonging to old 
surfaces. 
lf there has been any drifting of sand and dust over the dried 
suriace, or if the inequalities have been filled in any other way 
with a different material, we can study the modifications of an infilled 
track. The first point of importance to notice is, that the mud 
contracts enormously in drying; and it often happens that the top 
lamina shrinks before the next below it can adjust itself to the 
smaller surface, so that cracks are formed in the upper lamina 
only, which slides over that below by a kind of small horizontal 
iault. It then sometimes curls up by the more rapid drying and 
contraction of the upper surface, as toast bends, showing its 
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