236 
Geology of Sydney. 
what are called stalactites, from a Greek word signify- 
ing distillation or dropping. Whenever water filters 
through a limestone rock it dissolves a portion of it ; 
and on reaching any opening, such as a cavern, oozes 
from the sides or roof, and forms a drop, the moisture 
of which is soon evaporated by the air, and a small 
circular plate or ring of calcareous matter remains; 
another drop succeeds in the same place, and adds, 
from the same cause, a fresh coat of incrustation. In 
time, these successive additions produce a long, 
irregular, conical projection from the roof, which is 
generally hollow, and is continually being increased by 
the fresh accession of water loaded with calcareous or 
chalky matter ; this is deposited on the outside of the 
stalactite already formed, and trickling down adds to 
its length, by subsiding to the point and evaporating 
as before ; precisely in the same manner as, during 
frosty weather, icicles are formed on the edges of the 
eaves of a roof. When the supply of water holding 
lime in solution is too rapid to allow of its evapora- 
tion at the bottom of the stalactite, it drops on the 
floor of the cave, and, drying up, gradually forms, in 
like manner, a projection rising upwards from the 
ground, instead of hanging from the roof ; this is 
called, for the sake of distinction, a stalagmite. 
"It frequently happens, where these processes are 
uninterrupted, that a stalactite hanging from the roof, 
and a stalagmite formed immediately under it, from 
the superabundant water, increase until they unite, 
and thus constitute a natural pillar, apparently sup- 
