Animals which are not furnished with a
back-bone. For a further explanation, see "Vertebrated Animals."

ISOTHERMAL. Such zones or divisions of the land, ocean, or
atmosphere, which have an equal degree of mean annual warmth, are
said to be isothermal, from [Greek: isos], _isos_, equal, and
[Greek: therme], _therme_, heat.

IOINTS. Fissures or lines of parting in rocks, often at right angles
to the planes of stratification. The partings which divide columnar
basalt into prisms are joints.

IURA LIMESTONE. The limestones belonging to the Oolite Group
constitute the chief part of the mountains of Jura between France
and Switzerland; and hence the geologists of the Continent have
given the name to the group.

KEUPER. A German name for a member of the Upper New Red Sandstone.

KIMMERIDGE CLAY. A thick bed of clay, constituting a member of the
Oolite Group. So called because it is found well developed at
Kimmeridge, in the Isle of Purbeck, Dorsetshire.

LACUSTRINE. Belonging to a lake. _Etym._, _lacus_, a lake.

LAMANTINE. A living species of the herbivorous Cetacea or whale
tribe which inhabits the mouth of rivers on the coasts of Africa and
South America: the sea-cow.

LAMELLIFEROUS. Having a structure consisting of thin plates or
leaves like paper. _Etym._, _lamella_, the diminutive of _lamina_,
plate, and _fero_, to bear.

LAMINAE. Latin for plates; used in geology for the smaller layers of
which a stratum is frequently composed.

LANDSLIP. A portion of land that has slid down in consequence of
disturbance by an earthquake, or from being undermined by water
washing away the lower beds which supported it.

LAPIDIFICATION. Lapidifying process. Conversion into stone. _Etym._,
_lapis_, stone, and _fio_, to make.

LAPILLI. Small volcanic cinders. _Lapillus_, a little stone.

LAVA. The stone which flows in a melted state from a volcano.

LEPIDODENDRON, a genus of fossil plants of the Coal Measures,
intermediate in character between the Lycopodiums and coniferous
plants.

LEUCITE. A simple mineral found in volcanic rocks, crystallized, and
of a white color. _Etym._, [Greek: leucos], _leucos_, white.

LIAS. A provincial name for an argillaceous limestone, characterized
together with its associated beds by peculiar fossils, and forming a
particular group of strata, interposed bet