206 A LUMP OF CHALK AND ITS LESSONS. 



near neighbourhood of a coast afforded by 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 abyssal 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, instead of being very like the chalk, 

 were really very different. Even the ooze has not the 

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

 covering the ocean-basins 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 appreciable 

 portion of the red 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 the water of the 

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

 other delicate organisms which occur so beautifully pre- 

 served in the white chalk clearly indicate that its 

 deposition must have been comparatively rapid, and must 

 have taken place in a sea where 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 quite unlike that of the Atlantic depths. In- 

 deed, it is quite probable that the chalk sea may not 

 have exceeded some one or two thousand feet in depth. 

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

 explain its purity, and the very rare occurrence of drifted 



