192 NATURAL HISTORY OF THE FARM 
and sparkling, of a low and constant temperature, with a low 
oxygen content, and, owing to prolonged contact with the 
soil, with a high mineral content that varies much according 
to the character of the soil traversed. Deposits of sulphur 
and of iron are often formed about the mouths of mineral 
springs. But where the ordinary spring bubbles up, one 
usually sees only miniature deltas of clean-washed sand at the 
bottom of a limpid pool, which clears itself quickly after 
roiling. 
Spring water has a population of its own. 
Man and bird and beast are transient 
visitors who only quaff its waters; but 
there are other creatures, that permanently 
dwellinthem. They are things that cannot 
endure too great heat in summer or freezing 
in winter: things that like low equable 
temperature and partial shade. The most 
characteristic plant that grows in spring 
water is water cress (fig. 74); it was used 
by the pioneer to garnish his meat platter, 
and it is still so used. There are water- 
mosses, also suited to such a habitat, and 
many lesser algee of various kinds, both 
Fic. 74. A leat of green and brown. 
; There are animals, also, that live in 
spring water; such are the salamander shown in figure 75, 
and the brook trout, which does its best in water not warmer 
than 60° F., and, many other lesser creatures. Most of 
the great groups of animals are represented there, if 
by only a few forms: crustaceans; by the scuds, clamb- 
ering over and feeding upon the water-cress, and by 
asellus, wallowing in the soft bottom of the pools 
(fig. 20); molluscs, by little white clams (half an inch 
long, more or less), of the genus Spherium, furrowing the 
