PRESERVATION 23 



years, been confirmed by the observations made during the 

 fruitful voyage of H.M.S. Challenger in the Atlantic and Pacific 

 Oceans. 



Again, even in shallower parts of the old seas, where sand or 

 mud was once deposited fossilisation was somewhat accidental; 

 for some materials, being porous, allow of the percolation of water, 

 and in this way shells, bones, etc., have been dissolved and lost. 

 Thus sandstone strata are nearly barren in fossils compared to 

 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 " denuda- 

 tion;" 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 



