HOW EXTINCT MONSTERS ARE PRESERVED, 21 
shales and limestones, which are much less pervious. To take 
examples from our own country, the New Red Sandstone of the 
south-west of England, the midland counties, Cheshire, and other 
parts contains very few fossils indeed, while the clays and lime- 
stones of the succeeding Lias period abound in organic remains 
of all sorts. Even insects have left delicate impressions of their 
wings and bodies! while shells, corals, encrinites, fish-teeth, and 
bones of saurians are found in great numbers. 
Again, it must be borne in mind that the series of stratified 
rocks known to geologists is not complete or unbroken. They 
have been well compared to the leaves of a book on history, of 
which whole chapters and many separate pages have been torn 
out. These gaps, or ‘‘ breaks,” are due to what is called “‘ denu- 
dation ;” that is to say, a great many rocks, after having been 
slowly deposited in water, have been upraised to form dry land, 
and then, being subjected for ages to the destroying action of 
“rain and rivers,” or the waves of the sea, have been largely 
destroyed. Such rocks, in the language of geology, have been 
“‘ denuded ;” that is, stripped off, so that the underlying rocks are 
left bare. 
But the process of rock-making does not go on continuously in 
any one area. Sedimentary strata have been formed in slowly 
sinking areas. But, if subsidence ceases, and the downward 
movement becomes an upward one, then the bed of the sea is 
converted into dry land, and the geological record is broken ; for 
aqueous strata do not form on dry land. Blown sands and 
terrestrial lava-flows are exceptions ; but such accumulations are 
very small and insignificant, and may therefore be neglected, 
especially as they contain no fossils. 
In this way, as well as by the process of “ denudation” already 
alluded to, breaks occur; and these breaks often represent long 
intervals of time. ‘There are several such gaps in the British 
series of stratified rocks; and it is partly by means of these 
breaks, during which important geographical and other changes 
