MR. J. B. HANNAY ON SILICEOUS FOSSILIZATION. 235 



that a chemical examination was really what was required 

 to throw light on the question, 1 obtained from him some 

 siliceous fossils in which he was interested, and of which 

 he and Mr. J. Young, F.G.S., have published an account*, 

 and proceeded to make an examination of them by che- 

 mical means. The rods were samples of Hyalonema 

 Smithii ; and as they were in the fossil state siliceous, and 

 probably had been so in the original state, I took for com- 

 parison in my examination of them some rods of a recent 

 siliceous sponge. On examination of the fossil rods it 

 was seen that they might roughly be divided into two 

 classes : — I. rods partly transparent or opalescent, and 

 containing some chemically combined water ; and II. 

 rods opaque and apparently consisting of anhydrous crys- 

 talline silica, — both classes, however, containing many 

 samples stained brown by ferric oxide. Thus I generally 

 experimented with three different kinds, which shall be 

 referred to as 



I. Recent, quite transparent. 

 II. Fossil, opalescent. 

 III. Fossil, opaque. 



To find the general differences displayed by the three 

 kinds, several experiments with various chemicals were 

 first performed, placing all the three kinds in the same 

 vessel under the same circumstances. On placing the 

 three kinds in hydrofluoric acid, it was seen that the recent 

 rod was dissolved in about thirty minutes, the opalescent 

 in about two hours, and the opaque rod only partially dis- 

 solved after four hours^ immersion in the acid. Sometimes 

 the opaque rod dissolved irregularly, leaving protuberances, 

 and sometimes leaving a spongy substance covering the still 

 undissolved interior. On heating the three kinds in a four- 



* Annals of Nat. Hist. vol. xx. No. 119, p. 425. 



