70 CRETACEOUS ERA. 



chalk ; 4. Chalk with flints ; these two last being generally 

 white, hut in some districts red, and in others yellow. The 

 whole are, in England, about 1200 feet thick, showing the 

 considerable depths of the ocean in which the deposits were 

 made. 



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

 duction in such vast quantities was long a subject of specula- 

 tion among geologists. Some light 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 lived in the oceans of that era. Mr. Dar- 

 win, 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 explanation 

 of the production of chalk, if we admit some more recent dis- 

 coveries of Professor Ehrenberg. That master of microscopic 

 investigation announces, that chalk is composed partly of 

 " inorganic particles of irregular elliptical structure and gra- 

 nular slaty disposition," and partly of shells of inconceivable 

 minuteness, " varying from the one-twelfth to the two hun- 

 dred 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 large proportion 

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

 of the organic matter, being in some instances almost entirely 

 composed of it. He has been able to classify many of these 

 creatures, some of them being allied to the nautili, nummuli, 

 cyprides, &c. The shells of some are calcareous, of others 

 siliceous. M. Ehrenberg has likewise detected microscopic 

 sea-plants in the chalk. 



The distinctive feature of the uppermost chalk beds in 

 England is the presence of flint nodules. These are generally 

 disposed in layers parallel to each other. It was readily pre- 



