16 EXTINCT MONSTERS 
but such rocks as these do not contain fossils. They have been 
mainly formed by the action of great heat, and were forced up to 
the surface of the earth by pressure from below. As they slowly 
cooled, the mineral substances of which they were formed 
eradually crystallised; and it is this crystalline state, together 
with the signs of movement, that tells us of their once heated 
state. Such rocks are said to be of igneous origin (Lat. ignis, 
fire). But nearly all the other rocks were formed by the action 
of water—that is, under water,—and hence are known to 
geologists as aqueous deposits (Lat. aqua, water). They may be 
considered as sediments that slowly settled down in seas, lakes, 
or at the mouths of rivers. Such deposits are in the course of 
being formed at the present day. All round our coasts mud, 
sand, and gravel are being accumulated, layer by layer. These 
materials are constantly being swept off the land by the action 
of rain and rivers, and carried down to the sea. Perhaps, when 
staying at the seaside, you may have noticed, after rainy and 
rough weather, how the sea, for some distance from the shore, is 
discoloured with mud, especially at the mouth of a river. The 
sand, being heavy, soon sinks down, and this is the reason why 
sand-bars so frequently block the entrance to rivers. Then, again, 
the waves of the sea beat against the seashore and undermine 
the cliffs, bringing down great fragments, which after a time are 
completely broken up and worn down into rounded pebbles, or even 
fine sand and mud. It is very easy to see that in this way large 
quantities of sand, gravel, and mud are continually supplied to 
our seas. We can picture how they will settle down; the sand 
not far from the shore, and the fine mud further out to sea. 
When the rough weather ceases, the river becomes smaller and 
flows less rapidly, so that when the coarse débris of the land has 
settled down to form layers, or strata, of sand and gravel, then 
