CAYEENS AND THEIE CONTENTS. 
451 
few of the common metals are volatile, that one can hardly 
be surprised at metallic zinc being a comparatively modern 
discovery. 
Caverns in other rocks contain other minerals. In some of 
them native silver shoots forth from the walls in elegant strings, 
or the mixed ores of silver, lead, and antimony, with other 
less-used metals, are crystallized in various complicated shapes. 
Elsewhere, black, sooty walls mark the presence of manganese; 
and occasionally ores of copper, of extreme beauty, occupy 
niches in open spaces in slaty and granite rocks. But the great 
masses of valuable ores are not in caverns of the ordinary kind ; 
and the mineral contents of caverns are more likely to consist 
of an irregular floor of stones bedded in mud than of gems and 
metals. 
The vegetation foimd in caverns is less varied and less 
remarkable than the mineral wealth. A few ferns — some rare 
and curious, others common, but not less beautiful — are often 
pendent like a rich green fringe near the entrance and from 
the roofs and walls of those caves winch open to the sea, while 
a floor of varied and tangled sea-weed marks the extreme point 
to which the ordinary tidal wave has access. One fern in par- 
ticular, the sea spleen-wort (. Asplenmm marinum), though 
generally rare, is sometimes plentiful enough in such localities, 
and may be found in caves near Tenby, and elsewhere on the 
Welsh coast, and also abundantly in Devonshire, Cornwall, and 
the Channel Islands. Others have been found associated with it. 
Owing to the darkness that prevails during the day as well as 
at night in the recesses of caverns, there is little growth there of 
any land ; nor is it at all usual to find much vegetable matter 
drifted far in. All that grows or is deposited near the entry of 
a cave is pretty sure to be found also on the rocks of the 
same nature close at hand, and offers little or nothing that is 
peculiar in its mode of growth. 
Even in the animal kingdom the variety of species perma- 
nently inhabiting the recesses of caverns is very small. What 
there is, however, is curious enough; and these permanent 
residents deserve some notice before we pass to the principal 
subject of the present chapter, namely, an account of the 
creatures that have in old times made caverns their dens. 
Of all modifications and adaptations of structure, that of the 
very remarkable blind reptile found in several large caverns 
traversed by water, in different parts of the world, is certainly 
the most singular. This animal, the Proteus anc/uinus of 
naturalists, is a kind of salamander, or may be understood 
better as something between an eel and a tadpole. It is a foot 
long, of the size of a human finger, with four little legs, too 
imperfectly developed to be of any use as limbs. It lias no 
