UPPER SECONDARY ROCKS. 107 



occasioiially grejen or reddish nodules, which, upon 

 analysis, have yielded phosphates in great propor- 

 tions. Green sand is variable in its mineral char- 

 acters, sometimes consisting of loose silicious sand ; 

 at others it forms sandstones cemented by calca- 

 reous earth ; it also abounds in silicious concretions, 

 which vary from an opaque hornstone to flint and 

 chalcedony. 



In the United States the green sand forms a belt 

 extending from Sandy Hook to Georgia, and from 

 New-Jersey* to the neighbourhood of the Mandan 

 country. 



Chalk. 



The upper portion of the cretaceous group con- 

 sists of, 1. Chalk Marl; 2, The lower Chalk; 3. The 

 upper Chalk ; all of which are deficient in the United 

 Slates. 



Chalk is too well known to need a description. 

 It consists chiefly of carbonate of lime; has an earthy 

 texture, and is so soft as to yield to the nail. 

 Though generally white, in some parts of England 

 it is red ; and in Su^itzerland it is sometimes highly 

 indurated, resembling white statuary marble. The 

 greatest thickness of the chalk strata in England is 

 from 600 to 800 feet. The upper beds contain nu- 

 merous nodules, and short, irregular beds of flint ; 

 the lower chalk contains fewer flints, and is gener- 

 ally harder than the upper chalk, and is sometimes 

 used for building stones. The flint nodules are gen- 

 erally arranged in pretty regular layers in the chalk ; 

 they occur in detached concretions of various shapes 

 and sizes, and, when broken, sometimes are found to 

 contain beautiful crystals of sulphate of strontian. 



There has been a good deal of speculation in re- 

 lation to the origin of flint, some writers maintain- 

 ing that flint and chalk were capable of undergoing 



♦ Featherstonhaugh. 



