210 THE FRESH-WATER AQUARIUM. 
they show off the green plants much better; but they also 
show the green confervoid growths growing upon them much 
sooner than dark stones, and are much harder to clean after 
they have once become green. This difficulty of cleaning 
can be remedied by bering two sets of stones, one being 
buried in damp sand while the other is in use. Were the 
beautiful stones of almost fabulous brilliancy which cover 
the San Mateo beach, near San Francisco, as common on our 
shores, we should have a famous groundwork for the aqua- 
rium. After the layer of small stones has been evenly 
spread upon the bottom of the tank, we may arrange the 
rock-work in the centre in the following way, which seems to 
be a good one, because by it we avoid using cement, which 
makes a tank look altogether too artificial, and we get a 
strong piece of work giving sufficient shelter to the animals, 
and one that will not be likely to fall down and injure the 
glass of the tank. It consists, essentially, of a series of 
three stone bridges, the one above being smaller than the 
one below. If the tank is small one or two bridges may be 
all sufficient. -We take then two or more pieces of stone, 
having very rough edges so as to look more natural, and 
place them about a foot apart if the tank will admit of such 
a width, making a height of about two inches. Upon the 
tops of these pillars of support we place a thin flat stone, 
largé enough to rest firmly on them, and even lap over an 
inch or so on each side; then upon this flat stone we place 
the pillars of another bridge, having the next flat stone 
somewhat smaller than the other, and so on until we have 
made so many bridges that the top one will just rest upon 
the surface of the water. The distance between these flat 
stones may vary according to the fancy of the builder. The 
top stone makes a little Saad; and gives a chance for such 
animals as tritons and turtles to come out and sun them- 
selves or take the air. Another use which this top stone 
may be put to is this,—to support a small collection of 
kn plants, making a great ornament to the aquarium. 


