UPPER CHALK. 
87 
2. Chalcedony is often found occupying the 
hollows of hints, and is either mammillated, bo- 
tryoidal, or stalactitical. It sometimes forms the 
constituent substance of corallines, alcyonites, and 
other zoophytes, displaying in the most delicate 
manner the complicated structure of the originals. 
Its colour is of various shades of grey, azure, and 
pearl white, and in many examples it is beautifully 
translucent ; specimens are not uncommon in which 
the surface of the mammillated chalcedony has 
received an investment of crystidlised quartz. 
The stalactitical and botryoidal varieties are 
conhned to those nodules which retain a ])art of 
the original zoophyte. In some instiinces the hint 
passes insensibly into chalcedony ; in others the line 
of separation is most distinctly marked ; but in all, 
there is sufficient evidence that the chalcedony 
and quartz were deposited by inhltration, and must 
have passed through the substance of the hint. 
On this subject it has been remarked, that 
“ although, in the present compact state of the mat- 
ter of hint, it is not easy, though possible, to force 
a huid slowly through its pores, yet it is probable 
that before its consolidation was complete, it was 
permeable to a huid whose particles were hner 
than its own ; and that the particles of chalcedony, 
whilst yet in a huid state, being hner than those 
of common hint, did thus pass through the outer 
crust to the inner station they now occupy ; where 
they also allowed a passage through their own 
interstices to the still purer siliceous matter, which 
is often crystallised in the form of quartz in the 
centre of the chalcedony, and is so entirely sur- 
G 4 
