stone may be built up into a dark arch, or piled up after the fashion of a cromlech—one flat piece resting on two or three vertical pieces, so as to form a rude table-like structure. These may be fixed firmly in the places they are intended to occupy by means of Roman or Portland cement, which can be purchased at any building yard. The cement should be made into a stiff paste, and worked into the form required. Indeed, the rockwork may be wholly composed of such cement, especially if it is to have the form of an arch. The most important matter in the construction of rockwork is to give it a natural, rugged appearance, and to avoid loading the tank with superfluous weight. I have seen large shells and branches of coral in fresh-water tanks, and always thought the spectacle disgraceful to the owner. In a marine tank, such things are proper enough. Whatever is done should be made secure, the pieces of stone well embedded in cement, and the whole firmly united. The tank must be well seasoned, be frequently filled and emptied, to dissolve out any free salts before being put to use.
The Bottom must be composed of coarse river sand and small pebbles, the whole well washed before being introduced to the tank. Mr. Gosse condemns red sand and silver sand, as certain to stain the water. But I have two tanks now at work, both bottomed with such material, and the water preserves a crystalline brightness. I have also a marine tank, in which the bed is formed of common silver sand and garden pebbles: it has been in use nine months, and with no unfavourable results. In each case the sand was washed till the water could be poured away
