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174 the silts through their flocculation and precipitation, and that which is added in the form of snail shells abounding in the canals. The amount of these may be realized from the large numbers contained in the mud recently thrown out, as seen in the upper section of Fig. 95, where the pebbly appearance of the surface is caused by snail shells. In the lower section of the same illustration the white spots are

Fig. 95.—The recently removed canal mud, in the upper section of the illustration, is heavily charged with large snail shells. The lower section shows the shells in the soil of a recently spaded field.