94 VESTIGES OF THE 



tri2:)le alternation of sands and sandstones with clay;" 

 2, Gait, " a stiff blue or black clay, abounding in shells, 

 which frequently possess a pearly lustre ; " 3, Hard chalk ; 

 4, Chalk with flints; these two last being generally 

 white, but in some districts red, and in others yellow. 

 The whole are, in England, about 1200 feet thick, show- 

 ing the considerable depths of the ocean in which the 

 deposits were made. 



Chalk is a carbonate of lime, and the manner of its 

 production in such vast quantities was long a subject of 

 speculation among geologists. Some Hght seemed to be 

 thrown upon the subject a few years ago, when it was 

 observed that the detritus of coral reefs in the present 

 tropical seas gave a powder, undistinguishable, when 

 dried, from ordinary chalk. It then appeared likely that 

 the chalk beds were the detritus of the corals which were 

 in the oceans of that era. Mr. Darwin, who made some 

 curious inquiries on this point, further suggested that 

 the matter might have intermediately passed through 

 the bodies of worms and fish, such as feed on the corals 

 of the present day, and in whose stomachs he has found 

 impure chalk. This, however, cannot be a full explana- 

 tion of the production of chalk, if we admit some more 

 recent discoveries of Professor Ehrenberg. Tliat master 

 of microscopic investigation announces that chalk is 

 composed partly of 'inorganic particles of irregular 

 elliptical structure and granular slaty disposition," and 

 partly of shells of inconceivable minuteness, ''varying 

 from the one-twelfth to the two hundred and eighty- 

 eighth part of a line" — a cubic inch of the substance 

 containing above ten millions of them ! The chalk of 

 the north of Europe contains, he says, a larger propor- 

 tion of the inorganic matter ; that of the south, a larger 



