62 



ERA OF THE 



and Russia. The same system exists in Nor-h America, 



and probably in other parts of the earth not yet geologi- 

 cally investigated Being a marine deposit, it establishes 

 that seas existed at the time of its formation on the tracts 

 occupied by it, while some of its organic remains prove 

 that, in the neighborhood of those seas, there were tracts 

 of dry land. 



The cretaceous formation in England presents beds 

 chiefly sandy in the lowest part, chiefly in the clayey in 

 the middle, and chieflv of chalk in the upper part, the 

 chalk beds being never absent, which some of the lower 

 are in several places in the vale of the Mississippi ; again 

 the true chalk is wholly, or all but wholly, absent. In 

 the south of England, the lower beds are, (reckoning from 

 the lowest upwards,) 1. Shankland or greensand, '* a tri- 

 ple 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, 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 spe- 

 culation 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 were 

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

 curious inquiries on this point, further suggested that the 

 matter might have intermediately passed through the bo 

 dies 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 

 discoveries of Professor Ehrenberg. That master of mi- 

 croscopic investigation announces, that chalk is composed 

 partly of " inorganic particles of irregular elliptical struc* 

 ture 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 



