'266 Eepokt of the State Geologist. 



Z A few examples exhibit a singular condition of preservation, 

 and show a striking resemblance to a minute Favosite coral. The 

 radiating canals appear to be bounded by walls of silica, which 

 are pierced at regular intervals hy pores. The walls, however, 

 are seen to be double, preserving between them an imperfect cast 

 of the spicular framework. This framework evidently had been 

 overlaid by a siliceous deposit, then the colloidal silica of the 

 spicules replaced by calcite, and the whole sponge filled in with 

 the muddy sediment which now forms its matrix. 



In a large majority of specimens, the original siliceous elements 

 of the sponge have been imperfectly replaced by calcite. When 

 not decomposed, sections of these show well-developed walls, in 

 which the spicular mesh is indistinguishable from the secondary 

 deposit. 



All stages exist between specimens where the walls of the 

 radiating canals are well preserved and prominent upon the sur- 

 face, suggesting a globular form of Cbjstbtes, to others in which 

 the tube walls have been dissolved out, and the shaly matrix re- 

 duced to a soft and somewhat ochreous mass. Silicified examples 

 exist chiefly as casts. The radiating canals are represented by 

 radiating pillars, and these are connected with one another by 

 trabecular processes representing the pores which originally con- 

 nected the canals into a common system. In a few specimens, 

 traces of the original spicules seem to be preserved. 



I doubt if in any of the Lower Helderberg specimens, there is 

 a true replacement by pyrite, where the iron in solution replaces 

 the original material, molecule for molecule, thus preserving the 

 minute structure of the organism. However, in specimens com- 

 ing from a certain layer of the Shaly limestone, a crude pyritiza- 

 tion occurs, and such specimens can be nicely etched. Inasmuch 

 as the pyrite is granular and the structure no better preserved 

 than in other examples, it seems probable that small crystals of 

 pyrite are really embedded in or • cemented by silica. This con- 

 clusion is substantiated by a specimen, half of which is preserved 

 in this way as pyrite, and the rest appears as an encrusting shell 

 of silica in the manner above described. 



In a specimen to which reference has been made, and the only 

 one which preserves the original structure of the sponge, nearly 

 the reverse of this process seems to have occurred. The spicular 



