_Glacier._--A body of ice which slowly spreads or moves over the land
from its place of accumulation.
_Gneiss_ (pronounced nice).--A metamorphic, crystalline rock with
mineral grains arranged with long axes more or less parallel,
giving the rock a banded appearance. Derived from either igneous
or stratified rocks well within the earth under conditions of
pressure, and usually also heat and moisture.
_Igneous Rocks._--Rocks formed by the cooling and solidification
of a hot liquid material, known as magma, that has originated
at unknown depths within the earth. Those that have solidified
beneath the surface are known as intrusive rocks, or if the
cooling has taken place slowly at great depth, as plutonic rocks,
e.g. granite. Those that have flowed out over the surface are
known as effusive rocks, extrusive rocks, or lavas, e.g., basalt.
Volcanic rocks include not only lavas, but bombs, pumice, tuff,
volcanic ash, and other fragmental materials or ejecta thrown out
from volcanoes.
_Joints._--Nearly all rocks, except very loose surface materials, are
separated into blocks of varying size and shape by a system of
cracks called joints. They may be caused by earth-crust movements,
contraction during solidification of molten rocks, or contraction
during drying out of sediments.
_Jurassic._--The middle one of the three periods of the Mesozoic era
of geologic time.
_Lava._--An igneous rock which in molten condition has poured out
upon or close to the earth's surface, e.g. basalt.
_Limestone._--A sedimentary rock consisting essentially of carbonate
of lime which generally represents accumulation of shells of
organisms, but in some cases precipitates from solution. Often
impure.
_Loess_ (pronounced lurse with the r obscure).--A fine homogeneous
silt or loam showing usually no division into layers and forming
thick and extensive deposits in the Mississippi Valley and in
China. It is generally regarded as in part at least a deposit of
wind-blown dust.
_Marble._--A crystalline limestone, usually a metamorphic rock, the
limestone having been altered by heat, pressure, and moisture
within the earth.
_Meander._--To flow in serpentine curves. A loop in a stream. Most
streams in flowing across plains develop meanders.
_Mesa._--A flat-topped hill or mountain left isolated during the
general erosion or cutting down of a region.
_Mesozoic._--Next to the present era of geologic time. _ Metamorphic
Rock._--Any igneous or sedimentary rock which has undergone
metamorphism, that is notable alteration from its original
condition. (See Metamorphism.)