THE VOYAGE OF H.M.S. CHALLENGER, 
:iS0 
particles which boar distinctly the impress of the calcareous shells. Many, indeed, are 
internal casts reproducing with great distinctness and sharpness the form of the chambers 
in which the glauconite has been developed. Some of the casts have a brown colour 
presenting few of the characters of typical glauconite, and it will be seen that these 
pale-coloured or brownish casts predominate in the residue represented in PI. XXIV. 
fig. 3. When the residue of a Green Sand is shaken up with abundance of water, it may 
be separated by decantation into three distinct portions, the one composed mostly of the 
dark green grains of typical glauconite,^ the second in which the pale green casts pre- 
dominate,^ and the third consisting for the most part of the white, pale grey, yellow, and 
brownish casts,® 
When a sample of a typical Green Mud is hardened and cut into a thin microscopic 
section, it will be observed that a large number of the chambers of the Foraminifera and 
the areolar spaces in Echinoderm spines and other calcareous structures are empty, while 
others are partially filled with a small quantity of a brownish semi-transparent substance. 
This brownish material may fill one or two of the chambers, or may simply coat their 
internal surface ; in this way we may pass from shells only partially to those completely 
filled. It may frequently be observed that some of the smaller chambers have a 
distinctly green colour, while the larger ones are yellow or brownish ; other shells, again, 
are filled with a dark green substance, which presents all the characters of typical glauco- 
nite. A transition may thus be traced from the pale brown substance lining some of the 
chambers of the Foraminifera to the pale green substance that forms a complete cast ; 
this again passes into the dark green glauconitic grains. No external casts formed of matter 
deposited on the outside of the Foraminiferous shells have ever been observed, although 
exceptionally some shells presented a greenish coating of apparently glauconitic matter. 
Very frequently the reddish imperfect casts of the Foraminifera gave the reaction of 
phosphate of lime. In PI. XXV. numerous particles of glauconite are represented as they 
appear within the shells and in an isolated condition ; in many instances the shell 
appears to have been broken or thrown off by the continued growth of the internal 
glauconitic nucleus, the further growth eventually transforming the internal cast into an 
irregular glauconitic grain with a mammillated and furrowed surface ; the figures on this 
plate show all the transitions between these phases in the formation of glauconite. In 
many samples of Green ^luds and Sands are numerous minute particles about the same 
size as the glauconitic grains, and having the same furrowed and mammillated surface, of 
a brownish or green colour, but these from their internal structure are apparently highly 
altered grains of crystalline, 8chisto-cr}'stalline, and other rocks (see PI. XXV. figs. 2, 3). 
Finally, it may be added, there is often associated with the glauconite in the Green 
Muds from tlic shallower depths, an amorphous brownish green substance which either is. 
* Stc 8fl and 87. 
* See Analysis 85. 
* See Analysis 84. 
