HO W EXTINCT MONSTERS ARE PRESER VED. 2 1 



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 



