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POPULAR SCIENCE REVIEW. 
A mass — originally;, perhaps, a pulpy mass of lime-sancl, and 
clayey mud — is formed under water, and afterwards covered with 
other deposits. Exposed in this way to the influence of certain 
forces within the earth, it parts with much of its water, and the 
particles become compacted and often half crystalline. It 
retains/however, its character of a number of parallel beds and 
strata lying’ one above another. When such a mass, well 
hardened, comes to the earth’s surface, and above the water- 
level, it is inevitably cracked and broken, some of the cracks 
being wide and open, and others narrow and scarcely percep- 
tible. Once at the surface, all these crevices become water- 
channels, and some hill-side at a distance bleeds with in- 
numerable little streamlets, the result of rain falling on the 
earth, and penetrating below the vegetation and the soil into 
these crevices, and so through the mass of the rock. But 
water thus entering limestone contains carbonic acid, having 
passed through the atmosphere after distillation in Nature’s 
great alembic in the clouds, and water under such circum- 
stances freely takes up a certain portion of calcareous matter. 
Every drop that falls, then, removes its particles, and the narrow 
fissure, through which at first the trickling’ water could barely 
find its way, is soon enlarged, and becomes a watercourse. At 
frequent intervals, the' water reaches a bed of soft slimy mud, often 
found between two regular limestone strata, and its progress is 
then for the time interfered -with. It now acts mechanically as 
well as chemically, and scoops out a space that, under favour- 
able conditions, becomes itself a cavern. Forced to find its 
way occasionally between hard material, not favourably circum- 
stanced for mechanical action, and then suddenly reaching 
softer parts of the rock, a succession of chambers is formed, 
which communicate, indeed, with each other, but only by very 
narrow crevices. Thus it arises that limestone caverns are almost 
always both well ventilated and wet, and generally difficult of 
access. 
But other results are obtained from this origin. Water drips 
constantly through caverns once formed ; it often carries along’ 
with it a certain amount of fine mud, and, as it g’oes on, 
becomes charged with as much carbonate of lime in solution as 
it is able to hold. Whenever the water passes over a level 
surface, some of the mud is deposited, and whenever the draught 
of air that passes through the whole series of chambers is dry 
enough to admit of it, part of the dropping', or dropped water 
in the cavities evaporates, leaving behind a fi lm of limestone 
previously held in solution. In this respect, limestone caverns 
are entirely different from others, for the walls and floor, instead 
of being only mechanically worn by the rubbing of the water, 
are first eaten’ away by its solvent power, and then one part 
