106 UPPER SECONDARY ROCKS. 
Fi^. 30. 
A, Portland Marine Limestone ; B, Ancient Forest in the Dirt- 
Bed ; C, Lower Purbeck Beds of fresh-water Limestone. 
The upper series of the secondary formations con- 
sists of beds of green sand and chalk. In England, 
France, and Germany, this comprises a formation 
of great depth, computed to be at least one thou- 
sand feet in thickness, and its exclusively marine 
fossils prove that it was formed beneath the ocean. 
The lower part of this group has been usually 
called the green sand formation, because it abounds 
in green-coloured grains, which give, on analysis, 
silex, protoxide of iron,* alumine, and potash. This 
is separated from the chalk above by a bed of gray- 
ish blue clay, called gault, effervescing strongly, and 
containing some fossils. The green sandf is thus 
divided into the upper green sand, the gault, and the 
lower green sand. The lower green sand is very fer- 
ruginous, containing beds of sand more indurated 
towards the bottom with limestone in some locali- 
ties. 
The upper green sand is a mass of stratified sands, 
containing a large portion of the green grains, and 
* M. Berthier has analyzed the green particles, and found in 
every 100 parts 50 of silica, 21 of iron, and the remainder to 
consist of alumine, potash, and water. 
t Prof. Ren wick states that £reen sand owes its colour to a 
chloritoKS silicate of iron. 
