14 EXTINCT MONSTERS. 
object in writing this book is to interpret their story, and, as it 
were, to bring them to life again. Each one must be made to tell 
its own story, and that story will be far from complete if we cannot 
form some idea of how it found its way into a watery grave, and 
so was added to Nature’s museum. For this purpose we must 
briefly explain to the reader how the rocks we see around us 
have been deposited ; for these rocks are the tombs in which lost 
creations lie. 
Go into any’ ordinary quarry, where the men are at work, 
getting out the stone in blocks to be used in building, or for use 
on the roads, or for some other purpose, and you will be pretty 
sure to notice at the first glance that the rock is arranged as if it 
had been built up in layers. Now, this is true of all rocks that 
have been laid down by the agency of water—as most of them have 
been. True, there are exceptions, but every rule has its exceptions. 
If you went into a granite quarry at Aberdeen, or a basalt quarry 
near Edinburgh, you would not see these layers; 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 move- 
ment, that tells us of their once heated state. Such rocks are 
said to be of igneous origin (Lat. zgmzs, 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. agua, 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 sea-side, you may have 
noticed, after rainy ard rough weather, how the sea, for some 
