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 

 gradually 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 debris of the land has 

 settled down to form layers, or strata, of sand and gravel, then 



