SEDIMENTARY ROCKS 29 
samples may show well-rounded grains. These will 
probably have been water-worn. Other samples 
will contain angular grains. These give evidence of 
rocks broken up by quick changes of heat and cold, 
their little particles having been heaped together by 
wind. Such sands are common in dry climates. 
Other sands are composed of particles of carbonate 
of lime, derived from broken-up shells, corallines, and 
the limy remains of some seaweeds. These sands 
occur on the beaches of the coral islands of the 
Pacific. 
Sandstone is sand. cemented so that it has become 
a solid rock. The material which cements the grains 
together may have been laid down in water with the 
sand itself, or else have been introduced by water at 
a later period. The cement which binds the grains 
of sand in sandstone is of various composition. 
Some of it is a sort of clay, or it may be carbonate of 
lime, or a compound of iron or silica. 
Gravel consists of small pebbles that have been 
worn and rounded by the action of water. The 
pebbles may be rounded portions of any kind of 
rock, but they mostly consist of quartz, which, 
being hard and durable, can more readily sustain 
the constant grinding to which it is subjected on 
shores or in fast streams. When pebbles are larger 
than a chestnut, and they are gathered together as 
on a sea-beach, they form shingle. 
Grits are rocks formed of larger grains than those 
which enter into the composition of sandstones. 
They are practically sandstones, composed largely 
of coarse and sometimes sharp and angular grains 
of quartz. As you become more skilled in geo- 
