Appendix to Chapter V. 425 
in caves or in lacustrine deposits; and that not a cave or 
true lacustrine bed is known belonging to the age of our 
secondary or palzeozoic formations.” 
But perhaps of even more importance than all these known 
causes which prevent the formation of fossils, is the existence 
of unknown causes which make for the same result. For 
example, the Flysch-formation is a formation of several 
thousand feet in thickness (as much as 6000 in some places), 
and it extends for at least 300 miles from Vienna to 
Switzerland; moreover, it consists of shale and sandstone. 
Therefore, alike in respect of time, space, and character, it is 
just such a formation as we should expect to find highly rich 
in fossils; yet, “although this great mass has been most 
carefully searched, no fossils, except a few vegetable remains, 
have been found.” 
So much then for the difficulty, so to speak, which nature 
experiences in the manufacture of fossils. Probably not one 
per cent. of the species of animals which have inha'vited the 
earth has left a single individual asa fossil, whereby to record 
its past existence. 
But of even more importance than this difficulty of making 
fossils in the first instance, is the difficulty of preserving them 
when they are made. The vast majority of fossils have been 
formed under water, and a large proportional number of 
these—whether the animals were marine, terrestrial, or 
inhabitants of fresh water—have been formed in sedimentary 
deposits either of sand, gravel, or other porous material. 
Now, where such deposits have been afterwards raised into 
the air for any considerable time—and this has been more or 
less the case with all deposits which are available for explora- 
tion— their fossiliferous coiitents will have been, as a general 
rule, dissolved by the percolation of rain-water charged with 
carbonic acid. Similarly, sea-water has recently been found 
to be a surprisingly strong solvent of calcareous material: 
hence, Saturn-like, the ocean devours her own progeny 
