UPPER SECONDARY ROCKS. 107 
occasionally green 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 
States. 
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 Switzerland 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. 
