206 A LUMP OF CHALK AND ITS LESSONS. 



near iieiglibourlioocl of a coast afforded Ly the sandstones 

 of Saxony and Bohemia, indicates that this sea was a 

 mare clausum (in a geographical, not a political sense), 

 somewhat like the Mediterranean or the Black Sea. Now, 

 from the apparent similarity of chalk to the ooze forming 

 in the al)yssal depths of the Atlantic and the other large 

 ocean basins, it was taught but a few years ago that the 

 chalk itself was deposited in an ocean of similar depth. 

 The mare clausum theory, however, is of itself a sufficient 

 obstacle to the acceptance of such a view, since it is 

 impossible to conceive that a sea of such small dimensions 

 could ever have had depths at all approaching those of the 

 Atlantic. The Atlantic theory, if we may so call it, of the 

 chalk was, however, at once and for ever dissipated by 

 the researches carried on during the voyage of the 

 "Challenger." Those researches showed that the so- 

 called abyssal deposits, iastead of being very like tlie chalk, 

 were really very different. Even the ooze has not the 

 purity of the chalk ; while the large areas of red clays 

 covcriuL;' the occau-basias have no analogy in the latter. 

 Moreover, it has been proved that the ab^ ssal deposits are 

 laid down at a rate of almost inconceivable slowness — so 

 slowly indeed that even meteoric dust forms an apjireciable 

 portion of tlie I'ed clays ; while the ear-bones of whales 

 and teeth of sharks that strew the ocean-floor have lain 

 there so long as to have become coated over with a thick 

 layer of manganese precipitated from th(.' water of the 

 ocean. On the other hand, the remains of fishes and 

 other delicate org;inisms which occur so beautifnllv pi'e- 

 served in the white chalk clearly indicate that its 

 deposition must have lieen comparatively raj^id, and must 

 have taken place in a sea wliere there was abundance of 

 mineral matter either in suspension or solution. Again, 

 the fauna of the chalk, especially the sponges, is one 

 such as would be found in comparatively shallow seas, 

 and is cj^uite unlike that of the Atlantic depths. In- 

 deed, it is cjuite probable that the chalk sea mav not 

 have exceeded some one or two thousand feet in depth. 

 The great difficulty in regard to the chalk is, indeed, to 

 explaiu its purity, and the very rare occurrence of drifted 



