Geology and Natural History. 67 
other substances have not interfered with the perfection of the 
most minute of these structures. 
e 
aa have the cavities filled with a sedimentary limestone, an 
on Several fragmental specimens from Madoc are actually wholly 
nu Specimens present great difficulties to an observer; and _ 
peak no doubt that they are gets overlooked by collectors in 
sequence of their not being developed by weathering, or show- 
ng any obvious structure in fresh fractures, 
With regard to the canal system, the authors persist in confus- 
retions, and in likening them to dendritic crystallizations of silver, 
of lime. Ina r 
Oren as other than very imperfect, imitative. I may a 
7h Ss case 1s one of the occurrence of a canal structure in forms 
ich on other grounds appear to be organic, while the concre- 
ve te 
ous objections, I leave Dr. Hunt to deal. 
Ith regard to the proper wall and its minute tubulation, the 
8 
Morganic. With regard to the first of these positions. I ma 
Pet ey a8 angular crystals, closely pac ogether, while the 
spicular crystals of siliceous minerals which often oie 
cifica- 
and R ; 
‘ ‘af as é 
“sample of this; and whatever the nature of the crystals may be, 
* “ Quarterly Journal Geol. Society,” 1864. 
