204 ENGLISH CHALK DOWNS. 



Chalk as a whole was laid down at first in one 

 continuous slieet of white mud upon the bed of a 

 great inland sea, a branch of the Atlantic in far 

 distant times, which stretched like an old prime- 

 val Mediterranean right across the face of 

 Northern and Central Europe. Soft ooze of 

 almost exactly the same sort is still being depos- 

 ited in many parts of the Atlantic bottom, whence 

 it has been dredged up, in the yet plastic condi- 

 tion, from the depths of the sea, by the scientific 

 explorers of the Challenger expedition and other 

 investigators. This modern chalk, even now 

 growing up before our very eyes, consists chiefly 

 of extremely minute or microscopic shells, com- 

 posed for the most part of lime which the tiny 

 animals that frame the shells secrete from solu- 

 tion in the sea around them. Fragments of chalk 

 from the cliffs and cuttings of Kent or Sussex, 

 when examined in the powdered state under the 

 microscope, are found to be made up of broken 

 shells, exactly similar to these, each so minute as 

 to be quite invisible to the naked eye, but form- 

 ing in the mass the entire thickness of our English 

 deposits. In some places the chalk is loosely 

 interstratified with layers of flint, which derive 

 their origin from silicious sponges and other sim- 

 ilar creatures which lived in the same seas con- 

 temporaneously with the chalk-forming organisms. 

 At first, no doubt, flint and chalk, silicious ooze 



