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 
