80 ELEMENTARY GEOLOGY 
According to the cement which consolidates the sand 
grains, sandstones have been given different names: if 
the rock is clayey, it is called an argillaceous sandstone, 
or if the grain is very fine, it may be called an arena- 
ceous clay. If the cement is lime, as in some of the 
light-colored beds, the product is called calcareous sand- 
stone; if the cement is of iron, the rock is ferruginous, 
as in the brown and red sandstones; or the rock may 
have a cement of silica, when it is said to be silicious. 
In some sandstones there are many angular frag- 
ments, often giving the rock the character of a grit, 
which adapts it for use in grindstones; in other cases 
the rock splits easily m every direction, and it is then 
called a freestone ; and yet again, owing to the presence 
of many mica flakes, it cleaves readily in only one 
direction and is said to be shaly or micaceous sandstone. 
But while there are many different kinds, they are all 
alike in the fact that they are composed of small, 
visible grains of sand, usually quartz. 
Clayey Rocks. The fine clayey soil is an illustra- 
tion of a third group of the fragmental class, which 
may be called the clay rocks. The clayey fragments 
may be moved by the wind and gathered into beds on 
the land, as in northern China, where thousands of 
square miles are covered by a wind-blown clay, called 
loess (Fig. 60). They may also be accumulated in any 
body of water where the currents are quiet, —on the 
